That sounds kind of like Ba Sing Se. It's the largest city in the world with these huge walls. There are parts that are really beautiful, but some parts... not so much.

The Earth King lived there before the Fire Nation conquered the city, but even before then it was controlled by secret police called the Dai Li. The King didn't even know there had been a war going on for a hundred years. [ Judging by her tone, that's the kind of thing that's pretty hard to miss the memo about. ]