A keep was also known as a donjon, a French word. This word was probably altered over the years and its meaning changed so now the word dungeon means a small room used as a prison.

The castles are all built from a roughly uniform plan. A massive square tower with a square turret at each of the corners that project slightly. Each of the main faces of the castle has a flat buttress running up the centre of the wall for extra strength. The only parts that have decoration are usually the main doorway at the entrance and the chapel. At the centre of the keep are large halls. Some keeps have a dividing wall down the middle. Access to different levels and sections of the castle are by passages and spiral staircases built into the thick walls.

In 1078, William the Conqueror ordered the construction of a huge keep at the castle in London. Under the direction of Gundulf, a castle builder from Normandy, the tower known as the White Tower or Tower of London was built (pictured above). The size of the castle must have been greater than any building the citizens of London would have seen before and as William may have hoped probably struck them with awe. Although generally square in construction the White Tower has a circular turret to the north-east that has a circular staircase inside it and to the south-east there is a semicircular apse that contains a chapel. The southern side of the castle would of had a small tower at the entrance but this has been removed and not shown in the reconstruction at present.

Colchester Castle, the second of the keep-tower castles in England, was started between 1080 and 1085, again under the direction of Gundulf. After the defence of London had been provided for, William employed the new method of fortification where it was most needed next. The Danes had sailed up the estuaries of Essex within striking distance of London only a few years after the Conquest. To counter this threat, a new castle was built at Colchester. The chosen site was an old Roman fortification that still had substantial remains. The keep is larger than that at London and has walls that are 30 feet thick and foundations just as deep.