Legion20
I figure they probably should have blasted the monster tide with the anhilation, my guess is that it’s likely that the fortress draws power from the “troops” defending it which in turn lets it defend them. Or that’s what it was intended to do. If they kill the monsters then the fortress loses its amplifiers and can be whittled down until it’s destroyed.
It was literally explained just a couple chaps ago that Thaymos would wmeithe ruse a mana geyser to fuel his warp spells or wait until enough of the monster tide died to use their life force/mana to create one. I guess being a cursed object he can sacrifice his “hosts” or I’m this instance slaves, to repair himself like how Night and the three musketeers did