I thought I'd post this explaining the change to the game for City Worlds.
Really there should not have been a change but I found some bugs in the code that allowed City Worlds to be platformed and then once platform moved. Apparently this has been going on for a long time but due to the nature of playing the game and not knowing everything that was going on I was discussing something with a Nomad and found that they could not only platform City Worlds but move them once platformed.
Looking into the code I found a bug where the size of the planet is outside the range of the original game so the code had no filter. Originally the size of a world was limited between 0 and 11. Code checked 0, 10, and 11 to insure that the planet could not be platformed but did not check above 11. This was easily overlooked because of the way the code was written. It checked 11 and below to see if a planet could be platformed which kind of gets by you because 10 and 11 cannot be platformed, but that was filtered out later. Did not appear that planets above 11 could be platformed but the size being outside the range of checks was missed. The second part of the problem was that once platformed the size was not checked to see if the planet could move.
Really the status of the planet should have always been City World. Much like Ringworlds they are too big to platform or move. In fact like Ringworlds they cannot be cyber traded or destroyed in other ways either.
This was an oversight on my part as I did not expect the City Worlds to have a different size than what was in the original game. When the size was larger than 10 I had to scramble to make a lot of changes in a hurry. One or two was bound to be missed.
So blame me for the oversight. Just one of those things.