+ Peter V. Dell'Orto's blog Dungeon Fantastic for awhile, I've wanted to run my own GURPS Dungeon Fantasy megadungeon. I've been working on it on and off for a while and I hope it can be ready to go when I am done with Iron Gods.
I decided early on that I want the upper level to be a huge partially abandoned city. In my last DF game I had a few of these as sort of implied megadungeons, but where the PCs had very specific goals, and didn't explore very much (and I didn't have much planned out beyond the specific areas they needed to deal with). I think I like the city-as-dungeon concept, in general.
This map of Mal Tor'an (done with Campaign Cartographer 3 and City Designer 3) is a work in progress (I randomly generated it, and I still need to add some landmarks as well as key it). Each hex is 100 yards.
Mal Tor'an was built originally by giants and it's buildings are oversized as a result. It sits on an island in what's now a marshy valley (but was a mountain lake when the place was built). Below the city is an even older city built by lizardmen, and below that an abandoned dwarven city (newer actually than the two cities above it), and below that... Well, perhaps they delved too deep.