What is the Dark Tower in the novel, Doctor Who: The Five Doctors?

The Dark Tower is an imposing edifice. Built as the legendary Rassilon's tomb, it looms starkly above one edge of the Death Zone. It is psychically alive and conjures up realistic images from the Doctors' pasts as they search it. Because the tower is booby trapped in many places, the Doctors have to be alert and clever to reach its inner sanctum, which holds the answer to why they have been captured and brought together in violation of laws forbidding different incarnations of the same Time Lord from meeting one another except in the gravest emergencies.