When Steven Taylor crashed on the planet, the Mechonoids captured him and held him prisoner in the city. Two years later, the First Doctor and his companions arrived on Mechanus and were also taken to the city, where they met Steven. The pursuing Daleks entered the city and attacked the Mechonoids.
The Doctor and his friends escaped the Mechonoid City just before it, the Daleks and the Mechonoids were all destroyed. (TV: The Chase) The Mechonoid City was destroyed when the Daleks, led by a Black Dalek, used a atom divider to destroy its leg and the resulting explosion toppled the city and handed the Daleks victory against the Mechonoids. (COMIC: The World That Waits)