City Worlds have been added to the game so there are some rules you need to know.
1. Shipyards: There are limits to the number on each worlds according to size. 15 = 6, 20 = 7, and 25 = 8. All city worlds should have these on them when you got them.
2. Pop: There is a limit to 127 POP on each City World. Slightly more than a size 9 but that's all that will fit in the slot.
3. Marines and Heavy Marines: Still 100 times size.
4. Forts: Still 5 times size
5. ABM's: Limit is 255
6. Fighters: Limit is 255 till they are changed later.
7. MC's: 5 more than the MPOT of the planet if a miner installs them.
8. PC's: 5 times MC's up to a limit of 255
9. Torpedoes: 100 times size
10. Trading with them is the same as with a terraformed world.
11. Trade Centers, Ag Centers, Smuggling Centers, Spies, and Diplomats (if for some reason they become an NPC) can be placed on City Worlds.
12. Guerrillas can be landed on City Worlds and they can attack installations and military installations.
13. Military Bases can be landed on City Worlds.
14. They can be retooled like normal worlds. However warning. You must have 3 POP per MC on the world to do 3 or more levels per action. So they should all be fine unless you get a miner to install MC's on the 35 MPOT planet. In that case you can only do two levels per action. Best to retool this planet before asking a miner to put MC's on it.
15. Like Ringworlds they cannot be cyber traded although they can be looted, strip mined, or subjected to any other attack.
Enjoy.