At first the castle was leased to the king in the time of Henry II and then some time later it became the property of the crown. The castle was used by Henry II, King John and Henry III who between them improved the defences greatly. In the time of King John the Finham brook was dammed so that a moat was formed on the southern, western and eastern sides of the castle. This created a large expance of water that made the castle almost impossible to capture. Castles with a similar sized lake used for protection include Caerphilly Castle in South Wales and Leeds Castle in Kent. On the northern side of the castle a deep ditch was dug to provide defence. It was on this northern side that a gatehouse served as the main entrance to the castle. On the southern side of the castle the top of the dam also provided access to the castle. The narrow strip of land was protected by two tower gatehouses, one at each end.
The layout of the castle consists of a large outer ward surrounded by a curtain wall. Offset to the south-west is an area of higher ground where the keep and inner bailey are located. Several towers are built into the outer curtain wall including Lunns Tower and the Water Tower on the east side, Mortimer Tower protecting the southern entrance and Swan Tower on the north-west corner.
In 1243 Henry III granted the castle to Simon de Montfort but later Simon became Henry's enemy and was defeated and killed at the battle of Evesham. After the battle of Evesham Simon's son held out at Kenilworth Castle against Henry III for six months until finally running out of food and water. Edmund Earl of Lancaster, Henry III's son, was then granted ownership of the castle. The Lancasters were then associated with the castle including John of Gaunt, Earl of Lancaster, whose son became King Henry IV and during his reign the castle became a royal residence again.