there is a round keep in most castles and looks like the main part of the castle. but might not be the main part. and i cant find out what it is or was used for.
Round Keep Castle
The keep in a castle is the main fortress. The outer walls are the first part of the defences and everyone retreated to the keep after the outer walls were breached. The more powerful the owner, the more rings of defences he had, why some castles have more than 1 outer wall.
It was also the main place where the barons, kings etc lived and held thebanquets etc. When the keep fell the battle was well and truly lost …
It was used as a dungeon or fortress when fighting. Check Wikipedia.