This is a strange kind of thing... it's a solo game kind of thing, designed to create interesting dungeons for use in dunegeon-crawl rpgs. Basically, you work your way through various different ages, and the game tells you how things get organised.
It's really fun! That's the key bit. I've run through three dungeons now, one with the free version and two with the non-free. In each, there's certainly been a bunch of cool stuff that would make for a fun campaign. There's a lot of randomness that determines what happens in the dungeon, i.e. which monsters colonise which part and so on. There's also a series of events that each of the monsters will try each turn... dwarves will mine, dragons will hunt for food, etc. As time goes on, the humans living on the surface might even send expeditions or adventurers into the dungeon.
Dowler has come up with some great, evocative stuff in how the different groups behave, and the result dungeons have all been interesting - the results of all the events in the previous ages certainly give a lot of character to things! For example, one of mine ended up with a rich ettin lairing in the now-abandoned remains of an ancient demonic soul mill... nice food for describing in play!
Currently all my dungeons were on A4 paper (due to a lack of anything bigger) but yesterday I got myself an A3 pad, and I'm looking forward to playing again with a less cramped space (I think that may have led to my other dungeons 'finishing' quicker than they otherwise might have).
There's a nice example of how it works going on in a group dungeon making thread on story games, too.