I'm a fairly newbie player and haven't actually fought a titan before, but if I'd be tempted to let it go and see what happens. A popular tavern will attract a *lot* of petitions, so if you build 20-30 rooms associated with the tavern, you can fill them with mercenaries (who will only train and drink). Also keep in mind that the visitor limit defaults to *200*. Normally guests of the tavern will continue to make merry even when the fortress is under attack, but if a building destroyer starts knocking down the walls, they will help out. That gives you 2-3 squads of mercenaries, up to 100 visiting soldiers and up to another 100 visiting performers, which I guess is likely to be enough. You'll have to rebuild afterwards, but I don't think that's unreasonable.
I've been meaning to try a variant of this for a while -- A tavern/temple/library complex modelled on real life trappist monastaries. I particularly like the idea of having only mercenaries (apart from squad leaders) in your military. Everybody else is either an administrator, grower, brewer, bee keeper, tavern keeper, etc. Everybody wears pigtail robes, hoods and rope reed shoes. You can have a herd of cows and make cheese to serve in the tavern. Apart from disposing of garbage, your only trade is booze, cheese and religious figurines. Because weapons and armour will be prohibitively expensive you will need a small metal working facility. Again, I would tie it to a shrine (I'm imagining something like the Yasukuni shrine in Japan which historically made all of the katanas for military officers). I would mine only for stone blocks and rely on trade to get metals for smithing. And as another poster suggested, I think it should be mostly above ground, but should have an extensive system of caves for storing booze and cheese (you can even put those workshops in the caves). It can also serve as an evacuation centre for the civilians (as in real life!).