So I'm working on one of the first forts I've ever "planned out" instead of just building semi-haphazardly, and I'm pondering trading depot designs.
I have a main front drawbridge to the fort, and from there, I have two paths: One that leads to my traps of doom/military HQ/front door and one that leads to a trading depot/back door. I was thinking a good way to protect this backdoor was to "airlock" it between two drawbridges, one which would be up and one down at all times. (when there's a trade caravan coming, the outer bridge if down, and the inner bridge is up, letting the traders get to the depot. Once they get there, flip both bridges and I can carry stuff up the back entrance, to the now protected depot to trade)
Problem is I'm still somewhat new to Dwarven mechanics, so I'm looking for a way to control this. I read on the wiki somewhere, that one lever hooked up to two things that are "out of phase" won't work, so both bridges would end up matching, both up or both down. I know the easiest way to do it would be two levers, one for each bridge, and just make sure I pull them in the right order (So the down one goes up, before the up one goes down)
But does anyone have a way of building that kind of system into a single idiotproof(/Dwarfproof) lever? Optimally, I'd like it to raise the down bridge first before it lowers the up bridge, to make sure there's never a path available through the back door.
I was thinking of some weird thing with water and pressure plates and pumps, but I'm not good enough at Dwarfgineering yet to figure it out.