I'm trying to find a book that I read in (probably) the late 90s where the enemy would employ doppelgangers as one of their ultimate weapons to infiltrate, spy, and assassinate. I think that the doppelgangers had to kill their target in order to take on their shape/face.
The doppelgangers were not the main focus of the story. The enemy employed others as well... I vaguely recall a fireball and a doorway.
I think the main character stayed fairly central to one location (a castle) and that it wasn't a book with many, if any, journeys.
Unfortunately, I really don't remember a whole lot, and even the above was stretching.