Shra'kt'lor

Shra'kt'lor is the traditional capital city of the Githzerai in Limbo, ruled over by their god-king Zaerith Menyar-Ag-Gith. Detailed in the Planes of Chaos boxed set for Planescape, it consists of some 2 million active inhabitants and could support many more: it is intended to serve as the last refuse and final line of defense, should the githzerai's enemies ever invade Limbo and come after them.

As the book notes, such a "last stand" location is not so much a matter of necessity as it is an eloquent statement about the githzerai mindset.

Shra"kt'lor is an austere place with seven rings of high, thick, granite walls and massive iron gatehouses. Quarters inside the city are rather cramped, but as the githzerai aren't noted for mirth and gaiety, the lack of space suits them adequately. Foreigners are relegated to tents outside the main gates, where they cannot work mischief on the city itself or spy out its defenses. Outside the city, the Foreign Brigade - a band of mercenaries in the pay of the Great Githzerai - keeps an eye on things.

It's noted that there's plenty of arms and armor for sale even to foreigners, though attempting to outfit an army will likely get you negative attention. It's a decent enough place to provision before heading out into the howling mad chaos, though you won't find any entertainment--the city casts a pallid shadow even outside its gates.

In Planescape: Torment, the githzerai Zerth named Dak'kon was originally a respected denizen of Shra'kt'lor, until the day his crisis of faith caused a fatal weakness in its psionically-stabilized walls and resulted in the city being devastated; another githzerai NPC in the game will clarify that this was a result of a sudden, massive attack by the githyanki.

Whether or not Shra'kt'lor was destroyed by this assault is something that Planescape fans argue about. For what it's worth, it's mentioned in passing in the 3rd edition Manual of the Planes as still existing. In 4th edition, its place is taken by Zerthadlun.