The first Norman castles were forts in which the invading Norman lords took up personal residence. The earliest design, made of earth and timber, set the pattern that all castles followed after: the motte and bailey.
The motte was the basic tower that stood on a natural or artificial hill. Its core design was to have two stories with living space in the top. The first floor stored supplies.
The bailey was the palisade fence enclosing an area large enough to keep space between it and the inner building, but small enough to see all at once and maintain in defense. It had to be lower than the motte, for visibility.
As the motte turned into a complex building, it was often known as the donjon or keep. The old wooden motte was pulled down in sections to be improved with more stone and stronger construction. Early keeps could be round or square. The round design is known as a “shell keep” and it was based on concentric circles with rooms built into the inner circles. But more often, these early castle keeps were square towers.
Ardres Castle was built in France, in 1117 (about two generations after the Norman Conquest of England). It had three stories and a complex inner structure. The lowest floor was for food storage, but the kitchen was on both the ground floor and the second floor. The lord’s family generally lived on the second floor, which had a Great Hall. The same level also had an infirmary and the baron’s bedroom, as well as rooms for the more important servants. On the third story, there were many rooms for the baron’s children, more servants, and watchmen. Staircases and corridors connected all of the floors so that people could live under one roof as a community.
Rochester Castle, now a ruin, was a masterpiece of its time. The square keep was 38 meters high, and it was located in one corner of a large bailey. It appears to have included a full 3 floors, like Ardres. Several wells were dug inside the keep and had shafts built so that people on top floors could dip buckets into the wells without going down. The interior is now hollow, since the floors themselves were made of wood that subsequently decayed, in the 17th century.
In 1068, William the Conqueror returned to England from a tour of Normandy, and he began building a motte and bailey castle at Cambridge. The Domesday Book (tax registry) says that 27 houses had to be demolished to make room for the new structure, which had a bailey covering 4 acres. I think it must have been highly unpopular with the townspeople.
In Canterbury, the building of a motte and bailey castle cost the king 32 houses: he traded off 21 of them for land, and 11 more were torn down to make the ditch. The motte is still there, standing as a small, steep hill in the middle of a garden. It’s now known as Dane John Hill, because an antiquary decided to introduce a name that sounded like Danes built it, instead of its real name, Donjon Hill—because it’s where the keep was.