The continuing education program of the Royal City of Eoforwic presents

Medieval Military Engineering (or, " Have fun storming the castle")

by Hector of the Black Height

The aim of this presentation (a transcription of a presentation made to the Canton of Eoforwic in October 1997) is to enlighten you on a very specific type of military operation, common throughout our period of interest.


What is Military Engineering? Military Engineering is the application of engineering resources, knowledge and skill to a military purpose. There are several definitions of Military Engineering, but when looking at medieval campaigning one seems especially apt: Military Engineering is mastery of the ground.

What does this mean? In general terms, Military Engineers:

- allow friendly forces to move across the ground and obstacles (mobility tasks);

- deny enemy forces freedom of movement across the same ground (counter-mobility-tasks);

- allow friendly forces to hold ground with fortifications (survivability tasks); and

- force entry to enemy fortifications

So what? Translated into specific tasks, Military Engineers:

- bridge rivers and build roads;

- burn bridges across rivers and block roads;

- build castles; and

- break into them (housebreaking is at its heart a mobility task).

Through much of medieval history, there was no serious architecture outside castle and cathedral building. Only the nobility and clergy could afford Big Architecture. Just as building these huge structures became a specialty, so did tearing them down and defeating their special features. The other side to the architect's coin was siege engineering, which use a special skill-set to defeat the features built into castles by the architects of the time.

Siege engineers were known throughout medieval history, but they didn't necessarily have full-time employment. Through medieval history, there was no organized Military Engineer corps per se. In fact, some medieval armies had no "mynours" because miners, being important to the local economy, were exempt from military service.

Engineering operations are based upon available materials and technology. During the Dark Ages, vast quantities of Roman engineering knowledge were lost. Military Engineering didn't approach Roman standards for centuries thereafter. For example, the Romans employed two-armed ballistas; medieval European armies used single-armed onagers, because it's much easier to build and operate a single-armed torsion machine than to balance the torsion in a ballista's two arms. So what? Just because we know the Romans had the technology, don't assume medieval Europeans had it.

Having said all that, there really are two types of Military Engineering operations: offensive and defensive. As a rule offensive operations achieve mobility. Defensive operations are dedicated to counter-mobility and survivability. A principal strategic tool in defensive operations was the castle.


"Strategic tool" is an impressive-sounding phrase, but why exactly were castles built? Primarily, a castle was a secure lodging for a garrison. The presence of a garrison allowed the castle to act as a base of operations on a frontier, as happened in England in the Welsh marches or the Scottish borders. The result of this combination of presence and activity was the projection of political and military power. A castle was a visible, durable and tangible symbol of that power.

In addition, some castles provided coastal security. This is especially true during the Viking era. However, the security these castles provided was limited. A castle gave the local inhabitants a place to hide. It didn't project sufficient power to prevent the Vikings landing.

Finally, a castle might secure ground deemed to be of strategic importance. This could mean that a castle dominated (or was located upon) vital ground. This could be a harbour or port, a mountain pass or crossroad, or perhaps a location with significant resources (water, gold, etc.).

Now is a good time to skim through the evolution of the European castle, in very simplistic terms. First came the hill-fort, which is really the earliest type of European defensive fortification. This is a prominent feature, like a hill, surrounded by a wooden palisade. The hill offered the defender the uphill advantage and the palisade offered some security. It wasn't very sophisticated, but in the Dark Ages it sufficed.

The next step is typified by the beehive shieling, which is found in various places in Scotland and Ireland. This basically is a two-storey, round, stone tower with a door on the second storey and no access at ground level. When the Vikings came along to raid, the locals put up a ladder, climbed into the tower and then pulled up the ladder behind them. The Vikings could pillage to their hearts' content, but the inhabitants were safe, as the Vikings had neither the equipment nor the inclination to settle in and besiege a stone fortification.

The motte-and-bailey castle is the next significant step in medieval European fortifications, and was one of the most important developments in eleventh century architecture. A mound, or motte, was created. The motte was artificial high ground, and the ditch around the motte was a useful adjunct to the palisade, which secured the motte. A wooden keep was erected on top of the motte. This allowed defence in depth; the defender could fight from the palisade and fall back into the keep. This also meant that local farmers and livestock could be driven into the open area (or bailey) within the palisade, to secure important local resources for the defender. A motte-and-bailey castle was reasonably quick and easy to erect, which was an important consideration if you were invading a country and the locals were unhappy with you. William the Conqueror erected these in England in the 11th century; Oliver Cromwell erected them in Ireland in the 17th century.

The square keep was the next, logical step. Simply put, take a motte-and-bailey castle and replace the temporary wooden structures with stone, which is a lower-maintenance building material. It also is fire-resistant, which removes an important weapon from an attacker's arsenal.

The round keep or tower was the next step. A square keep has corners. In simple engineering terms, a corner is a place where the load of two walls meets at right angles, pushing outwards. Knock down the corner and the walls will be free to push outwards and collapse. Corners are very vulnerable to attack; make the keep round and there are no corners, which is a definite improvement. A lack of corners may also increase visibility at the base of neighbouring towers, which increases the ability of multiple towers in a castle to support each other with observation and fire. Overlapping arcs of observation and interlocking arcs of projectile weapon fire were (and still are) fundamentals of tactical operations in the defence.

The next and final step was the construction of soaring curtain walls, possibly joined to the keep, possibly as an outer layer of defence. The wooden palisade was replaced with stone walls, and then as architecture and masonry developed, these walls grew taller and taller. Height provided a distinct advantage against infantry armed with swords and spears. Arrows loosed from below had farther to travel to reach a target; arrows fired from the walls carried further. With a high wall, stones, lead and even garbage thrown from above became lethal weapons. The tall stone curtain marked the zenith of the medieval castle; imperious and impregnable, hard to defeat and easy to defend.

To build a castle the principal requirements were money, money and more money. Nobility wanting to build a castle would have to pay for a labour-intensive task at the cutting edge of the time's technology, which could take years to complete. Such a task demanded both skilled tradesmen and semi-skilled labourers. Quite a bit of farmland was needed to support those tradesmen, labourers and the soldiers who protected the work-site and eventually would become the local garrison. The land would support these mouths through actual food production and through taxes, used to pay wages. Reliable sources of stone, timber and other building materials also were needed. Without local materials, there'd better have been a river nearby to serve as a highway. Off-site materials would add considerably to the cost of construction; boats and boatmen would have to be hired.

Construction had to compliment location. A castle would require both a tactically sound site and a strategically useful site. What do I mean by this?

Tactics are the nitty-gritty of combat, the application of weapons to ground. The castle would have to be built in a location that would allow ease of observation of the surrounding ground from the castle, difficult approaches for likely attackers and natural obstacles (like a river) to enhance the castle's defensive features. This might mean clearing forest for several hundred yards around the castle, ditching and diverting water flow.

Strategy is the study of the political and diplomatic big picture. The castle would have to assist the dominant power (in most but not all cases the Crown) in achieving its political and military agenda. The castle would have to support other castles or garrisons, which would in turn support the castle with the possibility of relief in the event of an attack. All the tactical features imaginable are of no use if the castle is located in an insignificant spot that can be bypassed!


A significant spot, ground of interest to another nation or faction; such a location still may lead to conflict. In the Middle Ages, there was little or not rapid movement of troops; warfare was formal, with few surprises built in. The garrison of a castle would almost always have some warning (maybe hours, maybe days or weeks) of an enemy's advance, and they would have time to prepare themselves. The attacker would face a castle, gates barred and ramparts manned.

With some degree of notice, the besieged would gather all the local livestock inside the castle, plus as much grain and other foodstuffs as possible. Local farms, having been stripped of food, would be abandoned and possibly burnt to deny besiegers shelter and timber. Scorched earth is not a 20th century innovation!

In such a situation, the attacker's alternatives were to bypass the castle (leaving a garrison behind the attacking force, possibly on a supply line or withdrawal route, which never is a good idea) or to besiege and capture the castle. This assumes the attacker's aim was not just to capture the castle and eliminate the garrison as a threat or a political force.

To hold a castle in the face of a besieging force, there had to be a garrison force; somebody had to walk the walls and throw rocks at the people banging on the gate. The garrison required a reliable water supply found within the castle precincts. Without a supply of potable water a castle would fall in two or three days. Starvation is slower than thirst, but unless there was a reliable local food supply stored within the castle the defender was just as sure to surrender.

Beyond immediate needs, the castle's master would need money for wages, routine maintenance and so on, both before a siege and during the event itself. If the troops weren't paid (or promised pay, though promises start to ring hollow after a while) they'd desert, betray their employer -- more than one siege ended when a defender was paid off and opened the gate -- and generally be unreliable.

To capture a castle, a small army was necessary to lock the castle's defenders in and to keep the friends of the castle's defenders out. This army had to be fed, and if the besieged had used its time wisely all the food in the surrounding countryside would have been collected and was now sitting inside the castle, feeding the defenders. A besieging force had three options:

- bring a large quantity of food with them;

- have food sent to the siege from home, possibly across hostile territory; or

- forage farther and farther afield (which places foraging parties in progressively more and more vulnerable positions away from the site of the siege).

In any case, if food ran out before the castle had fallen it was past time for the besiegers to go home.

Forces trying to lift a siege are often overlooked when one considers siege operations. While trying to batter a castle wall down, defending one's rear was an important consideration. This required constant attention and significant resources. If the besieger neglected rear security, siege operations could suddenly change into a sandwich, with the besieger in the middle.

Conducting a siege demanded time; don't start a siege in October unless you like climbing ladders in a blizzard. Remember that the besieged were (relatively) cozy and warm inside a castle. The besieger had to sit on an open plain, making sure the castle and its garrison stayed bottled up. This would have been cold work in winter, especially as there'd be precious little food available in the surrounding countryside. If a siege hadn't broken the defenders well before the first threat of snow, the folks in the castle had won and the besiegers would have to withdraw and try again next year.


I've mentioned or alluded to several ways to capture a castle, such as starvation, thirst or intrigue. If these somewhat indirect methods didn't work, the attacker would have to consider physical attack on the castle itself. This attack could take any or all of three forms:

- below ground level (mining);

- at ground level (sapping, storming); and

- above ground level (bombardment).

Also, the attacker could see if pestilence (spontaneous or otherwise; using catapults to shoot nasty dead things into a castle is documented) would break the will of the besieged. However, this was seldom a sure bet and wasn't a popular tactic. Bacteriological warfare was a period concept, but -- as is the case today -- it was a two-edged sword.

One a siege began and the castle was invested, a waiting game began. The besieged sat within their walls and waited for relief. They'd hope the enemy would withdraw, but they'd prepare to repulse direct assaults on their fortifications. Defenders also watched for mining (which led to counter-mining) and sortied through sally ports to delay enemy operations.

So who is this Sally Ports person anyway? A sally port was a small, secure door in the castle's outer wall. Raiding parties would "sally forth" from these ports and attack the besiegers. These raids would attempt to set back the besiegers' schedule and slow the offensive siege process.

Sally targets included tools (metal was expensive back then!) which could be captured, labour-intensive works such as trenches and mines, siege engines and towers, labourers (preferably not pressed locals, but life happened), and imminent or developed breaches in the walls.

Before you can get over or through a wall you have to reach the wall. The attacker would deploy archers or catapults to clear the castle walls and neutralize the defenders. Trenches, shields and siege towers all could be used by attackers to move in relative safety towards the defenders. Siege towers and scaling ladders could be used to climb over a wall. All these means could be used at night or in bad weather to achieve some degree of surprise. Getting over a wall was easier said than done, though. A capable castle architect would make sure that the attacker found the ground offered no advantages. The fields around the castle would be open, to provide defenders clear bowshots. Moats and ditches would be dug to deny siege towers easy access to the walls. The road to the main gate would twist and turn, to deny battering rams a straight run. After a couple of unsuccessful attempts to get over the castle walls, the siege would probably settle into an attempt to defeat the walls themselves.


There were several methods of breaching a wall; these are familiar to most people through a combination of common sense and exposure to Errol Flynn movies. Battering rams could be used, as could picks and rock drills (not a pneumatic device, more like a large crowbar). Obviously these methods are labour-intensive and slow, which is what the besieged wanted; this gave the people on the walls more time to shoot arrows at the people trying to tear the wall down. Fire could be used against wooden palisades and against gates and the like, which is why castles made a rapid shift from wood to stone construction. Catapults were of limited utility, based on their trajectory and the relatively low velocity of their projectiles.

To breach the wall, siege engineers would pick a weak spot in the outer wall (bad stone, poor architecture, limited fields of observation and fire, possibly a sharp corner). They would try to build some sort of protective cover for the labourers working at the wall and then work parties would attack the wall at ground level. In this case the labourers would hope the wall wasn't rubble-filled, because if they broke open a hollow wall filled with loose rubble the workers would spend days and days shoveling dirt out to no real effect.


The besiegers also could attack below ground with a mine; they'd just hope there were no counter-mines, springs or moats waiting. The purpose of the mine was not to tunnel into the enemy's basement and allow a storming party to sneak inside. Rather, it was to dig out (literally to undermine) the castle's foundations at a key point and cause the collapse of a section of wall or a tower.

Mining was (and remains to this day) a very difficult and dangerous art. First, the miner would have to decide where to sink his mineshaft. This location of the mouth of the shaft was a compromise between the obvious desire to get close to the target and cut the distance to be mined and the need to get the mine entrance out of bow-shot and away from the threat of a sally. Once the mine was opened, the miners would try to conceal their operations. They would carefully haul away and hide the spoil (loose earth) from their excavation. A growing mountain of spoil was the defender's tip-off that he was being mined against, and the entrance to a mine would immediately become a prime sally target.

The next step, once the mine had traveled a few dozen yards, was to ventilate the shaft. This probably meant poking a long rod or tube through the ceiling of the mine and up to ground level above. A few small holes (so long as the mine wasn't terribly deep) would help ventilate the mine. A series of vents also would make sure the shaft was travelling in the correct direction.

Once the shaft had reached the enemy fortification, bypassing any moats or other underground obstacles, the miners would have to identify their target. If the castle was built on soil, it's likely the foundation would be resting on piles or some other obvious feature that could be located and attacked. If the castle was built on bedrock the miners would have to verify that the shaft had traveled the necessary distance in the appropriate direction.

Once the miners were content that they were under their target (a wall or tower), they would excavate under that target. As they excavated they would shore up the target with sturdy timber. Eventually they would produce a reasonably large, empty chamber, dotted with timbers holding up the roof (and that part of the castle).

At this stage the miners would bring as many flammables into the chamber as possible; dry brush, pitch, anything that would burn fiercely. There is a description of a herd of pigs being driven into the chamber and slaughtered, because lard burns well (obviously that was a short siege; in a long siege the pigs would have been somebody's dinner weeks before). On command someone tossed a torch into the assembled kindling and then everyone got out of there. The kindling ignited, the chamber turned into an inferno (assuming there were enough flammables and sufficient ventilation), the shoring timbers burned through and, if the miners had dome their job correctly, the walls above would collapse.

Froissart relates an anecdote from when a castle was under siege. The three commanders of the besieged castle were invited, under flag of truce, to a parley and a good dinner. Before dining, the besiegers invited the three commanders to join them down a mineshaft. The three guests reached the chamber, saw the shoring timbers and realized they were standing under their own castle walls. The commanders could foresee the inevitable. They had a convivial meal, returned to their castle and surrendered the next day. This spared their garrison the assault after the walls fell. It also preserved, intact, the castle for the besieger.

Earlier I mentioned counter-mines. If the attacker tried to mine under the castle, the medieval seismograph (a jar of water left in the deepest chambers of the castle; disturbances underground caused ripples, which were watched for) could detect the digging. The defenders would sink a mineshaft of their own to meet the advancing mineshaft. When mine and counter-mine met, the defender would flood both shafts (if a water source like an underground spring was convenient), set fire to both shafts, or fight hand-to-hand and then, if successful, collapse the other shaft manually.

A successful counter-mine did several things. First, it closed the mine before it could reach the castle. It destroyed scarce resources such as shoring timber. It wasted significant amounts of time, and killed trained miners. A fight underground or a blazing mine collapse would demoralize the surviving miners (it sure would demoralize me!), and a success like this definitely would raise the morale of the besieged.

Mining was clearly a long and difficult way to capture a castle. What about using trebuchets and catapults to breach the walls? These engines were inaccurate stone-lobbing machines; breaking walls needed hard, precise and flat shooting, preferably from long range. Catapults were useful to harass defenders but were not a major cause of casualties or structural damage. They definitely were not a significant threat to a well-built stone castle. Perhaps they might have been used to spread fire or disease within the walls. That was an attack on defenders' morale, not on defensive fortifications.

What if a castle's wall was breached? Hasty repairs or reinforcement would be made if possible. Some walled cities had inner walls, which would contain penetrations. These inner walls might include the back walls of small buildings erected within the castle compound.


If a breach was achieved and it couldn't be repaired or contained by the defenders, one of two things would happen. Either the nobility would surrender the castle to the people outside, or the defence would continue and the besiegers would try to put the castle to the sword.

Castle architecture was designed to keep the bad guys out, but the thought of a fight within the walls was always on the architect's mind. Defences within the castle were concentric. Layers of defence were independent. Towers on the wall as well as the main keep could be isolated as strong-points to allow continued resistance after the curtain was breached. Supplies would be decentralized within the castle with this in mind. Likely entrances such as gates were sited to favour defenders and limit attackers' options. Finally, gatehouses included murder holes in the roof for boiling oil, portcullises to break up attacks and so on, just like in the movies.

If the besieged chose to defend after having their wall breached, both sides were looking at a very nasty fight. The defence would be expecting the attack. The besiegers would be attacking into a narrow gap, across rubble that provided treacherous footing. The only way the attacker could expect to win such a fight was by employing speed and extreme violence. Such fighting would be fierce and casualties would be heavy, especially for the attacker; no quarter was expected during the fight or afterwards, which is why so many sieges ended in the sack of the castle or town. After fighting through a breach, the attacker wasn't in a generous mood.

Sometimes the castle would withstand a siege and the attacker would withdraw. Sometimes the walls would fall and the attacker would manage to fight through the breach and take the castle. As the victorious troops happily pillaged everything they could find, someone in charge would have to wonder what would happen next. This was a very important question, actually. The attacker had captured a castle with a broken wall. What good was that to a tired army in hostile country? If it took weeks or months to achieve a breach, would there be any food left on either side to support a garrison? Did the attacker capture any food from the besieged? This wasn't likely after a long siege; cats, rats and pigeons were documented as popular entrées after a few months without re-supply. In such a case the attacker might have to fill his wagons with loot, head home (as Henry V did soon after the siege of Harfleur) and hope he'd make it across hostile country to safety.


The race with technology never ends, and in the Middle Ages the architects were winning for a couple of centuries. Better architecture meant taller walls, fewer corners and better stone construction. For example, the original Tower of London, built shortly after the Norman Conqust of England, collapsed. Its replacement (the White Tower, completed at the turn of the 12th century and still standing) has walls 12 feet thick at the base to ensure stability. Such crude masonry was surpassed over the next two or three centuries by the graceful walls and efficient battlements of the classic medieval castle. The retreat of the Dark Ages led to new excellence in architecture and construction. Also it led to better siege weapons with longer ranges, but nothing that could top the stone castle.

And then came gunpowder (boom).

Three major military innovations resulted from the introduction of gunpowder:

- the petard (a bomb for blowing down walls and gates);

- the fougade (a buried charge, fired on command); and -- obviously --

- the cannon.

What's so great about a cannon, as opposed to a trebuchet or catapult? A cannon can hit one spot, from a distance, hard, with a flat trajectory, over and over. Consider the limitations of medieval siege engines; low velocity, high trajectory, relatively short range. The cannon solved all those problems. Suddenly the tall, solid stone walls everyone relied on for defence became a serious defensive liability.

To be fair, early cannons were very slow to load. Cannon barrels also had an upsetting tendency to burst on firing (especially the wooden ones. Yes, there were large cannons made out of wooden slats bound with iron hoops. Why do you think they call them cannon "barrels"?). Siege guns were heavy and awkward -- well nigh impossible, in some cases -- to move. However, once dug in and protected with fascines (bundles of sticks) and gabions (baskets of earth), cannons were almost invulnerable.

In the face of cannon fire, what could be done with existing castles? Some castle garrisons mounted counter-batteries of artillery up high, to increase their range. Others cut gun-ports low into their curtain walls, to bring direct fire to bear against besiegers. Neither of these stopgaps did anything to address the castle's biggest problem, which was its tall, flat curtain walls. With or without cannons on the battlements, a tall wall was still vulnerable to direct artillery fire.

If there was no real antidote for the cannon, what happened to the medieval castle? Some were torn down for their stone, which is not a sentimental option but was practical for the local serfs who wanted to upgrade their hovels. Some castles became status-symbol housing and subsequently museums, hotels and other tourist traps. After the fifteenth century and the introduction of mobile siege artillery, the nobility stopped building and living in castles. Instead, their homes evolved into stately manors of the type familiar to PBS viewers. Manors may be vulnerable to infantry attack, but they're much less drafty than stone castles.


Here ends the castle story. The tall curtain walls developed over 300 years suddenly had become a defensive liability. Defences that once held firm for months would now break open in days. What was needed was a design that would deflect cannon balls.

Gunpowder artillery ended the dominance of the castle. In areas with limited artillery threat, military castles of the motte-and-bailey type were built as late as the 17th century (by Cromwell in Ireland) and palisaded camps as late as the 19th century (the United States' cavalry forts in the "wild west"). In Europe, where there was an ongoing artillery threat, the soaring battlements of medieval military architecture were replaced by low, sloped walls and, eventually, by the glacis and counter-scarps of Sebastien Vauban's large star fortifications.

The evolution of the walled city also reflected the rise in importance of the urban middle class and the subtle shift in noble power, as the Renaissance and the Enlightenment supplanted feudalism. Cities were seen to be of value and thus required protection. As time passed, city-dwellers had the money and power to protect their own interests.

Thanks to advances in artillery and the evolution of society, defensive Military Engineering became less a question of castle architecture and more a question of urban planning. Offensive siege engineering remained a specialist military skill until the 20th century, in conjunction with the employment of siege artillery.

In conclusion, Military Engineering is mastery of the ground. Tactics is the application of weapons to ground. The castle was a tactical weapon of great power until advancing weapons technology (gunpowder artillery) rendered it obsolete. Artillery changed the face of European Military Engineering -- and in particular European military architecture -- forever.

So there you go: Medieval Military Engineering. Castle stand up; Castle fall down go boom.

Copyright 1996, 1998 Arthur McLean. All rights reserved.

Back to Hector's Index