Tomb of Rassilon
The tower contained a crypt where the physical remains of Rassilon supposedly lay. It had three entrances: a front entrance, a door on the roof, and access from caves below.
There were a variety of obstacles and boobytraps which made the journey into the tower complicated. One trap involved stepping on tiles that coincided with the mathematical concept pi. A party of Cybermen failed to recognise the danger and were destroyed by lightning-like bolts.
The Third Doctor and Sarah Jane Smith avoided these traps by making their way to the top of the tower by climbing across a wire. The First Doctor was able to work out the puzzles and pass through them. The Second Doctor encountered an illusion of his former companions, Jamie McCrimmon and Zoe Heriot. He recognised this as an illusion because the real Jamie and Zoe had no memories of their travels with him.
The First, Second and Third Doctors made their way to the tomb. After establishing a link for Borusa to transmat over, Borusa brought the brainwashed Fifth Doctor along with him, but the first three Doctors pulled the Fifth Doctor over to their side. Borusa, having tricked the Doctors, took the Ring of Rassilon, falling for Rassilon's ploy, and was trapped within stone. (TV: The Five Doctors) It was some time before he was given his freedom. (PROSE: The Eight Doctors)
There was a secret door in the Dark Tower which led to the Matrix and could be opened by the Ring of Rassilon. Rassilon possessed Leela and used her to lead President Romana into a Matrix Simulation of the Dark Tower. (AUDIO: Zagreus)
During the Last Great Time War, Rassilon housed his possibility engine in the Tower, which was made from a retro-evolved Borusa. He visited the Tower frequently to ask Borusa advice on how to proceed with his actions during the War. (PROSE: Engines of War)