The Fallen Fortress by R. A. Salvatore

R.A. Salvatore's The Cleric Quintet tells the tale of the scholar-priest Cadderly, who is plucked from the halls of the Edificant Library to fulfill a heroic quest across the land of Faerun.
Cadderly leads the combined forces of Carradoon and Shilmista against Castle Trinity, stronghold of his enemy Aballister. But another mission calls him on a journey leading into a past he hoped he would be able to forget.