One of the first citadels William the Conqueror built was, of course, in his new capital city. He was far from secure there; he needed to project power to the craftsmen who populated and controlled the city. Indeed, the City of London always acted like its own polity for centuries to come. Winning its approval—or glaring it into silence—was key for any king.
The Tower of London began as a wooden motte and bailey next to the Thames River. The site was pre-selected: it was an old Roman fortress, sited to control the river and use it for defense. To the west, there was still a Roman wall; to the south, the river. The bailey needed only two sides walled, which left plenty of timber to build the biggest donjon of them all.
So important was the Tower of London that during his later years, he started converting it to stone. The stone was shipped from Caen in Normandy, perhaps for security reasons, perhaps because they had found no native English stones that were as tough.
The motte would have been leveled, no longer needed, and a great foundation was dug. This donjon would have four floors including a true basement. Because the site was a shoreline sloping to the river, part of the basement—toward the river—was above ground.
The tower has been rebuilt since then, but it appears to have preserved the floor plan. The construction focus was on defensive features like thick walls, not comfort. Each floor has basically three rooms with stone walls: the smaller arched area that was the Chapel of St. John, and two other large rooms. There may have been wooden walls to subdivide the rooms.
The Chapel of St. John took up two floors, since it had a high ceiling with windows at the higher (safer for windows) level. Its roof was supported by pillars. The area below it would have been a crypt, but I don’t think we have records of anyone being buried there. The church with famous execution burials was a latter addition in the Inner Ward.
The Tower’s chief purpose was to be the King’s personal residence when he was in London. The top floor (where windows were safe) held his personal chambers in addition to a large Council Room. The same large space one floor down was a feast hall, and the King’s room one floor down was his public court. The floors had spiral stairs in their turret towers, so the King could move easily from his court to his private rooms.
When the keep was reconstructed in stone, they tore down the palisade walls of the bailey and rebuilt them, too. For the war technology of the English countryside, the castle was impregnable. It was also heavily guarded; it served not only as the King’s residence but also as the tax collection point for the rebellious city.
Many of the features of today’s Tower are newer, including the white facing stone that gives it the name “White Tower,” and the minaret-like turret roofs. Kings stopped living there by preference a few generations after the Conquest, but continued to use it as a secure home for centuries.
The White Tower’s floor plan is explained here.