Tavara's fortress is often called the second stronghold. It is protected by a variety of spells and creatures. The fortress cannot even be seen, felt, or accessed unless one is wearing an enchanted ring, which is found in the Inner Brotherhood stronghold. The fortress contains four chambers, each inside the last, and each with an entrance that is magically locked.
The outer chamber is a pool of water with some shallow islands that one can walk across. It is protected by Scylla. The next chamber is dry ground and protected by Undead. The third chamber has a stone floor and is protected by demons. The final chamber is a nice room containing only an elevated throne and a crystal ball (farsight). The lich Tavara hangs out in that inner chamber, with two golems.
The design of Tavara's fortress bears a striking resemblance to UrSylph's prison.