Mh... what would you guys think about an entire civ made of Forgotten Beasts that can siege from the caverns?
Two points to note:
~ The random composition of them, would need to be you know, not so random. As A critter of snow is... a joke. But a group of 10 steel or mythril, would be fortress ending without much real way to deal with it (1 titan you can lure into a trap and deal with, 10? Good luck).
~ Secondly, Syndromes. The deadly dust specifically (and fire a bit lesser so) are... absurd. If not choking to death, you're blown everywhere to your death. Imagine 10 titans with dust. Or 5 with dusts, 5 with fire, and the dust exploding the fire and igniting the entire damn cavern... actually, that'd be fun in a demented way
More so, I believe you would have to rigidly define the invaders into castes (Metal-Caste, Stone-Caste, etc) - obviously, making the 'deadly ones' act like the 'military leader' of their unit (so only 1-2 would exist at max size).
Similarly, the syndromes would have to be rewritten (so they are less... insane when there's 10+ titans at once). However, I think a general wide pool of non-super-lethal syndromes is great. Why? You beat them back... but suddenly your military/civs are beginning to pass out randomly, feverish, diseases running rampant causing madness (the classical titan syndrome issue of dust, it spreads like a bastard). Doesn't need to be lethal - but debilerating in some sense, would ensure even if you beat them, you won't leave unscathed, unless prepared.
Further, the size of siege would have to be VERY sensible. I could see about 10 at 'max size'. Even 10 is absurd, but still viable - especially if say 4-6 are 'soft' material (flesh, goo, etc - easy to 'kill' generally), 1-2 harder (stone or such), and 1 'metal' caste foe (presuming max size). So you can dispatch over half them fairly easily, but the others are a little tougher. This would further, force you to diversify. Axes are great!... but not against such variety. yet spears are not 'god mode' against the metal caste style titans, etc. Hell, you could even vary their actual 'size' (some 200 feet tall, some 15 feet - all 'titans/giants' in essence, which are what FB are).
If done right? It'd be brilliant. Hellishly difficult without the ol' cheap tactics, but for someone like who embarks in an undead, terrifying, thrall-cloud biome and finds it boringly easy? Perfect.