-you are on a LG plane
-you are in the direct domain of a God
-you are in a city that serves as a base for an army
-you are surrounded by high level clerics and paladins
I didn't want to close Celestia off from evil pc's, but be aware that security in the city is tight. Some NPCs will react according to your alignment. If your character is openly evil, have fun! Just don't pick a fight with the pallys

If you are secretly evil, you might consider staying away.
Also, some of the items there are blessed by the Gods of Celestia. UMD might get you around most restrictions, but you can't fool the Gods.