I will come along an English Fachbuch on medieval castles sooner or later. I would so far suggest either using "Palas" or describing it indeed as residential building or so.

The keep or castle keep is as LEO says it the Bergfried meaning a fortified tower. The Palas however is the main house of the castle housing the most important people of the castle - mostly the owners probably.

palatium (Latein/Latin)

:-)

#4: looking things up in the Wikipedia should be a more usual way of research for me. Especially if it confirms my guess. :)

IIRC palas is also a word of medieval sources. So it is still interesting how the main house of a castle was called in medieval English.