The planet Exxilon is home to an ancient culture, so scientifically advanced that they were spacefaring even before many planets had birthed life. On one of their jaunts through the universe, they happened upon Earth, where they taught the ancient Peruvians how to build temples. The Exxilons' crowning achievement was the construction of a living city able to protect, repair and maintain itself - it was designed to last through all of time. It was even given the equivalent of a brain, and this was the Exxilons' fatal mistake. The city decided that it did not need the Exxilons, so they tried to destroy it - but instead, it destroyed them, and drove out the survivors. Of the few Exxilons that remained, all feared the city - but while some chose to worship it and make sacrifices to it, others wanted to destroy it, for its influence would eventually destroy all the Exxilons.
The city is very large and brilliant white, towering over the surrounding terrain - in fact, it is so wondrous that it is considered to be one of the 700 Wonders of the Universe. It is topped with a very obvious beacon which lights up brilliantly at regular intervals, illuminating the entire city. The beacon is considered to be one of the city's roots, but a root in the air. It draws out electrical energy from its surroundings, which means that any spacecraft landing nearby are immediately drained of power, as are any electrical devices they use.
The Exxilon high priests have decreed that only they can approach the city, and anyone else caught near it is condemned to be sacrificed to it. To this end, the Exxilons use a kind of incense and force their sacrifice to breathe in the smoke, which acts as a tranquilizing drug. The sacrifices are then thrown into the tunnels underneath the city, where the ground roots of the city are to be found. These are a combination of both a support system for the city, and its weapons. They take the form of a kind of green eye on a long stalk, which lunge toward any lifeform they encounter. The touch of one of these creatures causes matter to burst into flame. The roots can surface anywhere above the ground or in tunnels below it.
The Exxilons have dusty-brown, leathery skin and large, glassy eyes. They usually cover themselves in dirt-colored rags, which make them blend in even better with the sandy surface of their planet. The planet Exxilon is rich in the rare mineral parrinium - it is as common as salt is on Earth - which is the only cure for a space plague begun by the Daleks, and hence very valuable. The Exxilons usually only wander the surface of the planet at night, but if they need to be out in the daytime, this is not problematic. They arm themselves with spears, axes, and bows and arrows, and their knives are poisoned so that a single stab will eventually kill a man. They do, however, take prisoners, and these are tied up and led to the high priest. If the prisoner is to be sacrificed, the Exxilons perform a chant beforehand. But the high priest is, understandably, considered sacred, and attacking him is one of the most terrible crimes on the planet. The group of Exxilons that do not believe in worshipping the city want to save the Exxilons from extinction, yet they are forced to live under the surface of the planet in secret places. Presumably because of their underground existance, some of these Exxilons have glowing stripes on their bodies. These include Bellal, an Exxilon who aided the Doctor and Sarah.
When the Doctor and Bellal try to broach the Exxilon city to shut off the power drain, they find that getting into the city requires passing an intelligence test - and moving further into it requires passing yet more tests, some fatal. One trap, resembling Venusian hopscotch (although it is unclear how a five-legged being would play it), sends a 7000-volt electrical charge through anyone who makes a misstep. Passing these tests proves that the adventurer has useful knowledge that could be added to the city's database. There appears to be an Exxilon manning the controls at the heart of the city, but his long-dead body disintegrates when the room is opened and the draught breaks the surface tension of his body. While the Doctor tries to engineer a nervous breakdown in the city, it produces antibodies - mutant-looking Exxilons that are not hurt by gunfire - to stop him. But the Doctor nevertheless succeeds, and when the Daleks plant explosives which destroy the beacon, the city dies, melting and screaming.
