The Great City of the Exxilons was a massive white building, geometrically designed and similar in appearance to the pyramids of the Aztecs. In the centre of the city was a tower with a glowing beacon which was its source of power. Also in the centre of the city was the control centre, which powered the whole city. It had no visible windows or doors and had sealed itself up after its creation.
The City was filled with traps which would kill anyone less intelligent than the City itself. Anyone more intelligent would have their mind scanned. Underneath the City were root-like structures which hunted by sonar and killed any trespassers. If these didn't deter intruders, the City would also produce humanoid antibodies.
The City absorbed all the energy from its surroundings to repair itself. All technology more advanced than bows and spears would fail, including technology as advanced as a TARDIS. Because of this, the City looked new, even after thousands of years. It also produced a mild electric charge which repelled dirt and grime. (PROSE: Death to the Daleks)
The Great City of the Exxilons was created by the Exxilons to show off their power and skills. They designed it so it was intelligent and could protect and repair itself. When it was completed, the City decided the Exxilons should be removed. It drove them out and sealed itself up.
Thousands of the years later, human and Dalek forces came to planet in search of parrinium. The Third Doctor and Sarah Jane Smith also came to the planet by accident. All of their technologies were drained of power by the city and they were trapped with the Exxilons. The Daleks modified their weapons, allowing them to enslave the humans and the Exxilons. The Daleks and the Doctor realised the City was the source of the energy drain. The Doctor and Bellal got past the traps using their wits, while the Daleks survived anything the City threw at them. The Doctor modified the City's computer, but he was chased out by the Daleks, who also planted a bomb on the beacon. Due to the actions of the Daleks and the Doctor, the city overloaded and destroyed itself. (PROSE: Death to the Daleks)