Zazz's moon was frequently pelted by meteorites.
Monektoni Shug crashed his spacecraft on the moon. A Servatron survived the crash and began building self-replicating robots. They soon developed a city on the moon. A robot was taken back to Zazz and began increasing its numbers to the point at which it became a plague. The Sixth Doctor lured them into a rocket and sent them back to the moon.
The city was the only structure on the moon; it took twenty generations to build. It had sloping pathways leading up to it.
The city had large announcement systems for emitting a recall signal; this was used to warn against impending meteorite showers. The robots would then return to the city and hide in the catacombs. The city was destroyed by a large meteorite. (COMIC: The Gift)