Since then I've tried making one above ground, but I'm not happy with the results. I'd like it better if water didn't leave permanent mud all over the surface. Shooting it in freezing weather doesn't work very well either, since globs will hit ice walls and build them back towards the fortification, jamming the setup I had. Of course it was only 1 level above ground, and shot at high velocity.
I'm assuming that building destroyers will attack floor bars. Too bad, that would make better drainage. I'm thinking of making pumps in 2 rows running the length of the corridor to keep the lane between the ramps dry.
The loop track has nothing to do with the launcher. It's the repeater for my shooting range, which is on the magma forge level. It leaves the door open 18 ticks (except when starting, it's open more than 100) then closes it, blocking every bolt before it can hit the goblin. I've gone through hundreds of bolts with no waste, and melting them gives back 250% of the metal you start with. Unfortunately the opening and closing causes lag.
I'm currently trying to find a decent embark to build my next design, even with dfhack prospect it's a pain to find sites with both sand and fire clay, and decent building space above and below the aquifer.