The Watchtower is outside, so follows the same lighting rules as the Greenhouse. Anything that falls off the edge dies (the only instadeath in the game). On the Watchtower are 8 Gargoyles and the Capcaun.
The Capcaun hits like a truck, has a massive amount of health, and even casts QUAKE 3. He may be the hardest boss in the game, but his slow speed makes him easy to kite. This boss fight should probably be one of the last things you do.
There is an easy way to kill the Capcaun: push him off the edge! Doing so won't grant you a ring, but you will get a huge amount of experience for very little effort. There are several ways to send him to his death: levitation, bat form, WHIRLWIND, confusion, or throwing.