If the title wasn't obvious enough, I'm going to be talking about the spoilers. Stop reading now if you don't know about the spoilers and don't want to.
I'm about to start mining the blue metal, as I have a legendary armoursmith ready to go and currently his talents are a little wasted. I know I have four spires and they extend very high (into the caverns tunnels, in two cases), so I can be reasonably sure of skimming a little off the top without triggering any unpleasantness, but sooner or later fun is going to happen, and I want to be reasonably prepared. I've never fought the spoiler people without losing the fort in the process, so I'd appreciate some advice.
My current plan is to devote an entire floor just to being a defensible entrance to the bluemetal mines. The trouble is, I'm not sure what qualifies as defense in this case. I know that..
* ..they can fly, so pits are not going to be much help.
* ..some carry syndromes, so at the very least I need a decontamination pool.
* ..they are building destroyers, so doors are useless.
* ..they don't drown and can swim in water and magma, so no using those.
* ..they come in both organic and inorganic varieties, so weapon variety is good
Beyond that I'm stuck. Will traps work, what about raised bridges? Would ballistae be effective? Any suggestions would be appreciated.