The castles were required so that the Normans could remain safe and in control. Major castles were built in or near large centres of population. On the edge of London William the Conqueror had a massive square keep constructed as a reminder to the Londoners that he was their king.
CASTLES IN THE MIDDLE AGES. Castles were primarily built during the wars of the late Middle Ages for the purpose of protection. ... The castle started as a simple wooden structure on top of a mound surrounded by a ditch. If the land was flat, an artificial moat would be constructed.
Castles were great defences against the enemy. However, when gunpowder was invented the castles stopped being an effective form of defence. By the end of the 1300s gunpowder was widely in use. The medieval castle with its high vertical walls was no longer the invincible fortification it had been.