My favorite battles in MASSIVE CHALICE are definitely the keep defense missions. The long, grey hallways contrast with the colorful tones of most other zones, and they are relatively closed in terms of movement.
The most important part though, is that they have an extra level of gravity about them: if you lose the mission, you lose the family that was stationed there. The Regent, the Partner, and all of the trainees are wiped out.
I’m not sure if the keep itself is destroyed, but either way, losing the bloodline can be crippling, especially if it is a family as crucial to your game as the Gisling bloodline is for my playthrough.