DOCTOR WHO IN DETAIL

The Death Zone is the legacy of the ancient Time Lord, and has caused shame to subsequent generations. In the Dark Ages the Time Lords used to kidnap creatures from other civilisations and set them down in an arena walled by an impenetrable force field to fight.

In the centre of the Death Zone is the Dark Tower. The Dark Tower also holds the Tomb of Rassilon where Rassilon, who possesses immortality, was locked away in eternal sleep. President Borusa learned that if he got into the Tomnb he could ask Rassilon for the secret of immortality, and used time scoop equipment to bring each incarnation of the Doctor, and some of his enemies, to fight in the zone. The Doctors made it to the Tower where they could disbale the equipment that stopped Borusa from teleporting in,  and they were to be present when Borusa gained the immortality he sought.