Tomb of Rassilon
The Tomb of Rassilon, also known as the Tower of Rassilon or the Dark Tower, was a large tower in the Death Zone on Gallifrey.
The tower contained a crypt where supposedly resided the physical remains of Rassilon. 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 such 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 lightening-like bolts that struck them.
The Third Doctor and Sarah Jane Smith avoided these traps by making their way to the top of the towere 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 to be an illusion as the real Jamie and Zoe had no memories of their travels with him.
The first five of the Doctor's incarnations made their way to the tomb. There they watched as the corrupt Borusa fell for Rassilon's ploy and was trapped within stone. (DW: The Five Doctors) It was sometime before he was given his freedom. (EDA: The Eight Doctors)
