structural:expansion
Warfare

As a static structure, castles could often be avoided. Their immediate area of influence was about 400 metres (1,300 ft) and their weapons had a short range even early in the age of artillery. However, leaving an enemy behind would allow them to interfere with communications and make raids. Garrisons were expensive and as a result often small unless the castle was important. Cost also meant that in peacetime garrisons were smaller, and small castles were manned by perhaps a couple of watchmen and gate-guards. Even in war, garrisons were not necessarily large as too many people in a defending force would strain supplies and impair the castle's ability to withstand a long siege. In 1403, a force of 37 archers successfully defended Caernarfon Castle against two assaults by Owain Glyndŵr's allies during a long siege, demonstrating that a small force could be effective.

Early on, manning a castle was a feudal duty of vassals to their magnates, and magnates to their kings, however this was later replaced with paid forces. A garrison was usually commanded by a constable whose peacetime role would have been looking after the castle in the owner's absence. Under him would have been knights who by benefit of their military training would have acted as a type of officer class. Below them were archers and bowmen, whose role was to prevent the enemy reaching the walls as can be seen by the positioning of arrowslits.

If it was necessary to seize control of a castle an army could either launch an assault or lay siege. It was more efficient to starve the garrison out than to assault it, particularly for the most heavily defended sites. Without relief from an external source, the defenders would eventually submit. Sieges could last weeks, months, and in rare cases years if the supplies of food and water were plentiful. A long siege could slow down the army, allowing help to come or for the enemy to prepare a larger force for later. Such an approach was not confined to castles, but was also applied to the fortified towns of the day. On occasion, siege castles would be built to defend the besiegers from a sudden sally and would have been abandoned after the siege ended one way or another.

If forced to assault a castle, there were many options available to the attackers. For wooden structures, such as early motte-and-baileys, fire was a real threat and attempts would be made to set them alight as can be seen in the Bayeux Tapestry. Projectile weapons had been used since antiquity and the mangonel and petraria – from Eastern and Roman origins respectively – were the main two that were used into the Middle Ages. The trebuchet, which probably evolved from the petraria in the 13th century, was the most effective siege weapon before the development of cannons. These weapons were vulnerable to fire from the castle as they had a short range and were large machines. Conversely, weapons such as trebuchets could be fired from within the castle due to the high trajectory of its projectile, and would be protected from direct fire by the curtain walls.

Ballistas or springalds were siege engines that worked on the same principles as crossbows. With their origins in Ancient Greece, tension was used to project a bolt or javelin. Missiles fired from these engines had a lower trajectory than trebuchets or mangonels and were more accurate. They were more commonly used against the garrison rather than the buildings of a castle. Eventually cannons developed to the point where they were more powerful and had a greater range than the trebuchet, and became the main weapon in siege warfare.

Walls could be undermined by a sap. A mine leading to the wall would be dug and once the target had been reached, the wooden supports preventing the tunnel from collapsing would be burned. It would cave in and bring down the structure above. Building a castle on a rock outcrop or surrounding it with a wide, deep moat helped prevent this. A counter-mine could be dug towards the besiegers' tunnel; assuming the two converged, this would result in underground hand-to-hand combat. Mining was so effective that during the siege of Margat in 1285 when the garrison were informed a sap was being dug they surrendered. Battering rams were also used, usually in the form of a tree trunk given an iron cap. They were used to force open the castle gates, although they were sometimes used against walls with less effect.

As an alternative to the time-consuming task of creating a breach, an escalade could be attempted to capture the walls with fighting along the walkways behind the battlements. In this instance, attackers would be vulnerable to arrow fire. A safer option for those assaulting a castle was to use a siege tower, sometimes called a belfry. Once ditches around a castle were partially filled in, these wooden, movable towers could be pushed against the curtain wall. As well as offering some protection for those inside, a siege tower could overlook the interior of a castle, giving bowmen an advantageous position from which to unleash missiles.