I think I found one or two things, but nothing really related to making it more fun. I agree it needs some balance issues, making demons stronger but that has been suggested more than just basically my thing, adding 'fluff' to it. I figure these floating clouds of substances could drift up into your fortress. I think that after the initial rush, they could perform 'sieges' instead of one or two coming up at a time. There could be slave pits, (intelligent creatures shackled to a rock inside a caveish thing) you could destroy their chains (thinking interact with weapon or teeth if it is a rope) and recruit these creatures (elves, dwarves, goblins, kobolds, a few animal men) to your cause, letting them journey with you as meat shields. Or if you get a dwarf down there, you could free them and they will gladly run away. Again, most of these things would have effects (swimming into river of clown blood or poison wouldn't be harmless, same thing with clouds of boiling vapour) but mostly add aesthetic if you survive it, and even a bit of a reward for survive it (though momentary) to go exploring. My main belief is it shouldn't be possible to conquer the circus. For a few ingame days, you might be able to get a dwarf or cat down there, if you somehow manage the first rush. Afterwards, it should be eerily quiet, and then suddenly a large number of them siege and come from all edges of the map in the circus, and make a beeline for any creature in the underworld not shackled to the wall, undead, or a clown, and kill them. Next, the congregate near the entrance to the underworld, and suddenly they enter, charging through your fortress.