Roger Zelazny 
The Changing Land [EPUB ebook] 

The Changing Land picks up shortly after the events chronicled in Dilvish, the Damned. Dilvish must gain entry to Castle Timeless if he is to destroy his nemeses Jelerak. Unbenounced to Dilvish, Jelerak is also seeking to gain entrance to his own castle as his apprentice has blocked him out while attempting to lean the castles secrets so that he may become lord of Castle Timeless.
The castle is protected by a lethal, chaotic, ever changing land. In a race against time and reality itself both Dilvish and Jelerak must survive the maze like changing lands and then somehow survive whatever traps await them and perhaps even the elder gods themselves may intervene if the whim strikes them. A breath taking grand adventure!