Mordor

He-he, so many mods/cities have been made (are being made), but it doesn't really affect the balance, except perhaps in the Phoenix mod, where the global AI assistance is too exaggerated. It's very difficult to play on S and M maps because you reach the enemy's territory on the second week, and there's a "defender" with all skills at level 20, a bunch of secondary skills, and a large army (for that day)... okay, I'm getting carried away.

Added after 53 minutes
Personally, I think such a city would fit perfectly into the world of Heroes of Might and Magic. Nazg on a wyvern because wyverns are very similar to the winged friends of the Nazgûl.