Hi all,
I have an idea I'm testing out, but a distinct shortage of building destroyer test subjects at the moment:
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Essentially, we have an entrance stairwell on the left (e.g. exit from the pit under my dodgeme catwalk), a long (25+) corridor running along the middle (truncated here for brevity), and a split airlock on the right: the top corridor has a drawbridge and the bottom one has a retractable bridge above it. Both are linked to the same repeater, so at any given instant only one exit is open. A good repeater can switch them exactly every 100 ticks.
If I understand correctly, even the fastest creatures in the game can't travel 25 tiles in the time it takes a bridge to close, so the gobbos and their trolls will always see a path but never actually reach an exit. Line the corridor with traps, put some fortifications, and you're golden.
The pedestrian version has to be an exit rather than an entrance so it doesn't trap dwarves, pets, caravans, etc. None of those has any business being at the bottom of a dodgeme pit, so they should be safe. Also, demoralized enemies stop trying to enter your fort, but they will always try to escape.
A second version is reversed: the split airlock is an entrance (only accessible to fliers) which just happens to [appear to] be the shortest way into my fort. This time, no friendly creature has any means to get there, so menacing spikes on a repeater are a very nice choice for dealing with trap avoiders.
These grinders also have the advantage of always welcoming new visitors: no matter how many are inside, there's always room for one more.
One question tho: I'm getting mixed signals on whether building destroyers can take out retractable bridges. Does anyone have experience with this?