From the introduction:
The keep was built on the remains of a pagan temple, the upper chapel of which was torn down and de-consecrated by the priests of the Prophet. The main temple, below ground, was sealed off, for the place reeked of evil.
After the keep was abandoned, the baron killed, and the keep lay empty again, a magic-user found the place. He set up his lair past the crypt of the temple, using his powerful magic to carve it out of the rock. He sealed the original entrance after an encounter with the walking dead, and made a tunnel that leads out to a secret cave in the woods outside.
The magic-user died in a botched experiment, blown apart by his own failure. His lair has remained undiscovered and untouched since then.
The temple has also been left largely untouched, as the few who have entered it became spooked or slain by something unseen. It has been left alone since the bandits took over the keep and had a bad experience in the main temple. It has not been entered for several years.