Sentinelspire (Forgotten Realms, the Citadels)

To save the future, he must embrace his past...

A ranger and his apprentice are captured by a group of assassins who demand their help in overthrowing their leader and the ruler of the fortress of Sentinelspire, the Old Man of the Mountain. Although the ranger doesn't want to help, he discovers the Old Man's plans may involve the master druid who gave him a new life--and the destruction of all he now holds dear.

A series that centers on the citadels - castles, keeps, fortreeses, and watchtowers - of the Forgotten Realms world, each book in The Citadels series is a self-contained fantasy adventure.