http://agtp.romhack.net/fns.htmlCool stuff. Think Nethack with graphics, essentially. (Go look up roguelike games if you don't know what that is.) You don't get to go back to old levels, but you do get to store stuff in a few warehouses for use on the next run. There are also quests you can do that affect the world in general so that the next run is a bit easier: getting companions, extra shops, extra items, etc.
The basic game is still much easier than Nethack, partly because all items are identified. After you beat the main and 2 extra dungeons you get to face the 100 level challenge where all items are unidentified, and many items are added, most of which mess you up. There's also a fun mini-puzzle dungeon where you learn basic principles about the game.
The appeal of the base game is the standard roguelike learning curve. The two extra dungeon introduce new ways to play. The final dungeon just shows you how cruel the RNG can be.
"Oh...a Power House. I didn't know those existed."