Ollanhar Series by Robert E. Keller

Lannon Sunshield and his friends have found a new home--the dreary, cursed tower known as Ollanhar where the Dark Watchmen once dwelt. They vow to restore and defend the ancient keep, even as an old enemy returns stronger than ever and creatures of darkness close in on them. Meanwhile, King Verlamer continues to expand his colossal, brutal empire, and Dremlock Kingdom remains his primary target. Yet the key to Dremlock's survival may exist in distant lands.

The quest for the Green Flamestone continues, taking Lannon and the Divine Knights ever deeper into strange lands and greater peril. Meanwhile, the cursed walls of Ollanhar Tower close in on a young Knight, threatening to strip away his sanity.