D&D cities should be like icebergs
The majority of what happens in a fantasy metropolis should be "beneath the surface": whether literally (sewers, catacombs and ruins below the city streets); or metaphorically (secret plots and political machinations). I've mentioned the city of Marchion (in Splintered Peace by David Chart ) before. The megadungeon beneath Mage Hill is mostly implied - it is up to the individual DM to populate that location with specific dungeon levels and denizens. The site of Marchion was settled in the distant past, but no one knows by whom. Sometimes strange artifacts are unearthed by people digging foundations for houses, and many think that Mage Hill isn't a natural feature. However, these inhabitants, whoever they were, were long gone before the first recorded attempts to settle the site. Local folk tales tell of the buried complexes they left behind, but no one knows whether these stories have any substance. Aboveground, most of the mages' towers still stand, ...