Keep
A keep is a strong central Tower which forms the heart of a Castle. It is usually a central square Tower house. Often the keep is the most defended area of a castle, and as such may form the main habitation area, or contain important stores, or the armoury. An earlier word for this, still used for some medieval monuments, is donjon.
Some castles were in fact, no more than a keep. Often these are simply referred to as "tower houses".
The keep commonly houses the lord or king that lives there and usually marks the highest point of the castle. Some of them contained donjons, or dungeons.