Deepcity was the advanced weapons research and development facility for the Landoran Alliance. Its location was the greatest secret in the Alliance.
Deepcity appeared be built in an isolated valley on an unnamed planet. In fact, it was inside a hollowed out asteroid. To maintain the illusion it was on a planet, the cavern inside the asteroid had an artificial sky with a 'sun' in the day and 'stars' at night. There was also artificial gravity, which the Fourth Doctor speculated was made by a layer of stabilised neutronium on the floor of the cavern.
When it was revealed that Averon had been decimated years ago, and the war with the Averon Union since then a sham, the people in Deepcity decided to shut everything down. However, Max told them he would keep it open to create more synthoids like himself to maintain the peace in the Alliance. (PROSE: A Device of Death)