Fig 1. If a strong castle had to be taken by assault, it was necessary to use all the siege-weapons then known in order to break down its defences.
Fig 2. Keep of a mediaeval castle: this is a stone tower, built high up on a rock, with a splendid view. Such places are rare in England, but very common in some parts of Europe, English castle builders did what they could to find safe sites with a good view for their keeps.
Fig 3. The battlements could take various forms, but the purpose of these was always to give the defenders protection, and yet allow them to set fire at the enemy, wherever he was.
All over Europe are the remains of stone castles, and of course, there used to be many more, of which no traces are now left. In Germany alone there were once over 10,000 castles. From the eleventh to the fifteenth centuries castles were the most powerful 'weapons' in warfare, because to defend a place was easier than to attack it. This remained true until the invention of gunpowder, when gun were built which could knock down a stone castle. The first castles in England were built of wood, on top of earth mounds. From a wooden tower the sentry could see an enemy approaching from a long way off. When the enemy reached the castle he had to climb the mound before he could attack the fortress itself. But these small fortresses were not easy to defend, and as they were made of wood attackers could set fire to them.
In the late eleventh and twelfth centuries large stone towers became the chief buildings of castles (see Figure 2), like the Tower of London, built by William the Conqueror. But a stone tower was too small to hold many people for long; and if the attackers could dig under the walls, they could undermine them. They would dig a tunnel, support the roof and walls with timber props until they had undermined a good length of wall, and then burn the props so that the mine and the wall over it collapsed. Many castles had an outer enclosure. This was eventually surrounded by one or two circles of 'curtain' walls. At frequent intervals in these walls were towers where defending garrisons lived, and from which they could fire arrows or pour boiling oil on attackers at the foot of the curtain wall. Later, the top of the curtain wall was improved: high battlements were built with slits through which defenders could shoot without being shot at. Overhanging battlements allowed the defenders to look down on the face of the walls and shoot at or pour oil on their attackers (see Figure 3). Castles were often besieged in the eleventh, twelfth, and early thirteenth centuries. Full-scale sieges were not so common after that, because castles were too strong. But most English castles withstood several sieges between the twelfth and seventeenth centuries.
The problem with besiegers
A castle could be taken by surprise if the attackers could reach it before the defending garrison knew they were there. But castles were usually placed at good look-out points; all trees and every form of 'cover' were cleared from the neighborhood and a constant watch was kept. A castle could also be taken if there were treachery in the garrison. But it was not at all easy to get messages into a well-defended castle. Eventually, most castles had to surrender when their food supplies ran out; usually they were built where the garrisons could be sure of plenty of water, so thirst was not a problem. But it was possible to build up a large supply of stores in the castles' cellars and so it was easier for the defending garrison to find enough to eat during a long siege than for the besieging army.
If a strong castle had to be taken by assault, it was necessary to use all the siege-weapons then known in order to break down its defences.
Refer to Figure 1.
1. A battering-ram, under a shelter which protected it from the defenders. This was only useful against doorways or weak parts of a wall, or after undermining the walls.
2. The trebuchet, which shot stones at and over the castle walls. This was like a huge see-saw; one end shaped like a spoon, was slowly lowered and then suddenly released, so that it shot into the air and threw with great force a stone which had been placed in it.
3. The tower, which enabled a force of knights to attack the walls without having to climb them. This was very effective – until the defenders burned the wooden tower.
4. The shelter, which enabled the archers of the besieging army to get close to the walls and pick off the defenders, to get a clear stretch of wall for the assaulting parties.
5. The scaling ladders, which the attackers could climb. They would be lucky to get them up safely if the wall had not been cleared of archers, or broken down by miners or stones from the trebuchet. But once over the wall or into the breach, an attacking force could often quickly open a gate or lower a drawbridge and let their comrades in.