Fallen Fortress — R.A. Salvatore

Cadderly is charged with waging war - between the combined forces of Carradoon and Shilmista Forest and the fiends of Castle Trinity, the stronghold of his nemesis. Instead he sets off with a small force of his own to conduct a smaller campaign, using his increasingly powerful priestly magic. Neither the journey nor the battle go as easily as planned, and Cadderly must come to terms with both his friends' expectations and with a past he'd almost rather forget.