The keep (round tower) in Windsor Castle, England.

Almost all castle construction began as a single tower, to which other defensive works were later added. William the Conqueror had erected a wood fortress at Windsor, but this stone tower was finally completed under King Edward III (1312-1377). Crusaders had adopted the Arabic use of round towers, which are stronger than square ones, and the practice quickly spread through Europe.

The keep usually housed the noble and his family and was the strongest and most heavily fortified part of the castle.

Keeps were sometimes built round and sometimes square. Square keeps made nicer rooms, but round keeps were easier to defend. The keep In it the great hall was located. The great hall was the heart of the castle. It was used for family dinners, banquets, games, dancing, and entertainment and sometimes a courtroom.