There are your standard fantasy RPG settings and then you have settings like The Horrendous Heap of Sixteen Cities, which is anything but standard. There is no way my own words will do it justice, so I'll borrow Dylan's:
Extending above a haze of reeking steam rise sixteen peaks of garbage magically transported from sixteen different cities. It spreads, like an ever-growing fungus, across the landscape, encompassing and corrupting nearly fifty square miles. Hideous flies, crows, and vultures circle the piles, perpetually avoiding garbage falling from magical portals thousands of feet in the air. Giant rats, skunks, maggots, and other manner of repugnant beasts scuttle about the surface, surviving off the offal. Underneath, giant worms crawl through the debris. Periodic explosions reform the horizon. Some cultures call it “Sheoal”, others “Kol Katta”. All, however, use the common vernacular “The Heap”. And everywhere its name is synonymous with “Hell”.
As you can see, far from standard. Also, far from large. It is basically a micro-setting - an area to be placed within a larger setting. As such, and due to the fairly unique and special nature of the HHoSC!, this is something that you will probably need to plant seeds of knowledge fairly early in the campaign. PCs would have heard of something this unique, at least as a legend or rumor, so to spring it on your players out of the blue would be horribly unfair. Still, nothing will truly prepare them for experiencing the real thing.
The HHoSC! is very much a sandbox, even if its a small sandbox, and it provides the GM with a number of hooks to get the party to (and into) the HHoSC! It is appropriate for most levels, although low level characters will have to tread very carefully.
This isn't a plop and drop type of adventure, but if you prepare in advance, it should give your party a unique and fun experience.
[4 of 5 Stars!]