I've been thinking about running a campaign entirely in a buried city that would be sort of a cross between Pompeii and Atlantis - a highly advanced civilization buried by catastrophe. For additional interest, I figured the catastrophe should be their fault - backlash from a spell that wiped out an enemy city. My suggestions:
Make it a bubble city like aco175 suggested, but the bubble has failed. Large portions of the dome have collapsed, so that large chunks of the city have been demolished, and others are crumbling. Tunnels might connect some parts, and others would be totally isolated. It doesn't sound like you want your party to go very far, so you might also have different parts of the city linked by portals or teleportation circles that require a ritual to activate. The PCs might figure out what it does, but without the ritual it just becomes a tantalizing possibility.
If the PCs are just visiting, I'd only give them access to fairly mundane sections of the city. Apartment buildings, individual residences (Roman-style villas, for instance), bathhouses, maybe temples. In fact, you could probably find floor plans for Roman ruins online, and arbitrarily collapse sections. Also, a buried coliseum could be pretty cool. Wherever they end up, I'd include signs of fighting. After all, a buried city doesn't have much food (well, it's more fun if they don't have create food and water, anyway). After burial, there would have been lots of deaths from starvation, and plenty more from people fighting over food.
For that matter, some of the inhabitants might have found a way to use the portals/teleportation circles to hide themselves just out of phase with the city. In this state they'd be timeless in the sense of not having to eat, drink, sleep, or age. Some of these inhabitants might be recoverable (if the party magic users are persistent about tampering with the portals, say), and some of them may be (partially?) insane from millenia of confinement. They definitely should not be able to offer anything like a coherent history.
With that much magic and bad history, you might also consider doing flashbacks. Maybe ghostly scenes from just before or after the catastrophe, or even interactive flashbacks.
I'd say, too, that no high magic city is complete without summoned creatures. Formerly bound (now berserk) elementals are always fun, but let's not overlook demons trapped in magic circles. All they need is some hapless adventurer to break the warding, and the shapeshifting ability to trick them into it. If it follows a flashback of survivors stowing themselves away in teleportation circles, even better.
I think I would not include undead. Too easy, too familiar. Not unless they were going to explore the city long enough to find the sorts of inhabitants who would have been in a position to turn themselves into liches.
Eh, that's what I've got for now. I've got a pretty specific buried city in mind, but hopefully some of it's useful to you.
