Question about mechanisms.
I've been trying to set up an filtering entrance to the fortress with a pressure plate and a hatch.
The idea was to put a plate linked up to an open hatch which would shut if enemies came near, but would remain open for dwarves.
That's the idea anyway.
In reality, it seems I can only make the pressure plate open the hatch, not close it.
Is there any way to link up a pressure plate to make a hatch close, or is there another solution for dwarf filtration? I'd like to let my dwarves come and go as they please but keep goblins and undesirables from entering. A low-maintenance entrance.
It's, uhh, complicated.
The simple answer is to use a bridge instead of a hatch. When triggered, a hatch opens. When triggered, a bridge retracts (or raises). So you have inverse effects, assuming you want the hatch to open to permit access (you can also build it so a hatch serves as a floor, rather than a ceiling, in which case a hatch would do the trick; use a hatch over an empty channel for this effect.)
It gets complicated by the fact that bridges only trigger after a delay. So you'd have to put the pressure plate a long ways (10 tiles or more) from the bridge.
Alternatively, you could build a water logic inverter system. This would consist of a pressure plate that is normally dry, leading to a trigger task (opening the hatch) until it gets covered with water, caused by an opening hatch above it dropping water on it. This is confusing and complicated, and it would still lead to a delay.
I guess the simplest version of your system looks something like:
side view:
__^H____>XXX
XXX\_XXX<___
H = hatch
^ = pressure plate
> = down stair
< = up stair
X = wall
\ = ramp
_ = floor
where the pressure plate is set to trip on all weights but no civilians, and is linked to the hatch. That opens that hatch, blocking access to your stairwell. If you design it well, this should be proof against goblins, but trolls or ogres will just march up and destroy the hatch and then nobody can get in and you have to rebuild the whole setup.