because they wanted to
1st. motte and bailey castle 2nd. square keep castle 3rd. shell keep castle 4th. concentric castle
The Motte and Bailey style of castle had two major weaknesses. Frequently the motte was too weak to support the castle. It would simply topple over. Second, an enemy could tunnel through the motte and attack the castle that way. If the castle were build first, the tunnel could reach the castle without the defenders knowing where the attackers were until it was too late. The shell keep style did not have that problem. In fact, it could be made round so that cannon balls would glance off.
A Square Keep castle usually sits on a hill. It only has 1 wall of defence, whereas a concentric castle usually has about 3 thick walls. Concentric's also have a keep in the middle that has a wall 2.5 metres thick!
Stone
a drawbridge and archers
Restormel castle
One Shell Square was created in 1972.
Shell keep castles are round rather than boxed. A castle that forms a circle is a perfect example. There are many in England.
Round Keep Castles were a type of castle during the medieval times. They were not built on a motte, unlike shell keeps. They were an adaptation of the square keep castles except they had round or polygonal walls. Medieval builders couldn't make the round keep castles as big as the square keep castles, so they were often cramped for space.
They did not change from shell keep castles to square keep castle, they existed at the same time. Most shell keeps were a result of replacing the wooden towers on top of the man made Mottes (erected as a quick way of making a castle) with stone buildings, as they were man made it was often thought they would not take the weight of keep with very tall thick walls like the Tower of London so they made shell keeps, with thinner walls. To build a substantial keep they would have to demolish the Motte. You would have thicker stronger walls with a square tower, but the extra height on top of the motte is also a defensive advantage, and it woule be cheaper.
A shell on a castle is the round part on top. Since Norman Motte and Bailey castles were first made of wood, the castle was not sturdy enough to support the shell.
A shell Keep is a rounded shaped castle which was built in the midevil times !!!!by Lauren xxx