Square Keep
The first stone square keep in England was built by William the Conqueror and was the Tower of London. The square keep had improved from the motte and bailey as it had thick stone walls with very narrow windows. It also had spiral staircases made anti-clockwise so that defenders could easily swing their swords at their enemy. There was a large moat surrounding the keep to stop enemies crossing. This castle was easily destroyed by attackers tunnelling underground and burning the wooden supports at the corners of the keep. This caused the walls to collapse. ( See picture of a square keep castle).
Corfe Castle
Round Keep
William the Conquerors descendent King Edward I built these castles to defeat the welsh in 1272. He killed the Prince of Wales called Llewellyn.
Edward I
The round keep was better than the square keep because all round vision was possible, tunnellers couldn’t destroy it as it had round corners, the walls were thicker at the base and the towers leant over the wall so objects could be dropped on the enemy.(see plan of keep castle below).
At this time siege and powerful long range weapons were made e.g. the mangonel and the trebuchet. The mangonel was a catapult that fired a number of heavy rocks in one go and was powered by the tension of ropes. It was however difficult to aim. (see picture).
The trebuchet was very accurate. It fired one heavy rock at a time. The rock was placed on the trebuchet arm which knocked into the rope and sprung off at great speed. (see picture).
Siege towers were developed to help attackers climb high walls. The siege tower was covered in thick leather to give protection to the attackers. Heavy battering rams were used to try and destroy the castle walls. One successful method of attack was the siege. During a siege the attackers would surround and stop anyone leaving or entering the castle. Eventually the defenders would run out of food and have to surrender.
Concentric Castle
The concentric castles were also built by King Edward I who made them at the end of the 13th century. This castle was a great improvement from the round keep. These castles had two or three curtain walls more than 6 metres thick. The inner walls were higher than the outer wall so men on all walls could fire at the enemy. The main defensive features of this castle were moats, ditches, drawbridges, portcullises, gate passages, parapets and arrow towers. The moat stopped attackers putting battering rams or siege towers against the walls. Drawbridges could be closed quickly as they were hinged. The portcullis ( a heavy grid) strengthened the entrance. It could be raised and lowered quickly by machinery in the gatehouse above. In the entrance hall was the gate passage where defenders would drop rocks and boiling water on attackers from the roof and sides. There were also murder holes in the roof where burning oil was dropped onto attackers. It was hard to destroy these castles with the weapons described above. Edward I built many of these castles in Wales at the end of the 13th century. (see picture of Beaumaris castle on Anglesea built in 1295 and the plan of a concentric castle below).
Platform Castles
King Henry VIII built the last castles in 1500. He built most of them along the English coast as he feared that the French might invade. This castle could not be destroyed by any weapons. It had many defences to withstand gunpowder. It had a very small entrance, the towers were built below the outer walls so that cannon balls would not hit it with power, the walls were 5 metres thick and the clover leaf shape could absorb cannon balls. These castles were very difficult to attack. They were defended by revolving cannons on their roofs. The only disadvantage was these castles were very dark and uncomfortable inside.
Examples of this castle are Deal Castle (below),Walmer Castle and Sandown Castle.
Conclusion
The reason castles changed overtime was because weaponary was getting more advanced so kings had to make stronger and larger castles which would withstand powerful weaponary (especially gunpowder).