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[Sutremaine]

The framerate on my current fortress is slow enough that I want to get this project done without any mistakes, but it would still be quicker to build this now than to start a new fortress. It's a collapsing-bridge goblin trap, but it uses water-sensitive pressure plates to reset itself. If it works, I won't have to do a thing during a siege unless there are traders around.

I've thought it through and decided that it does work... several times. So I'm running it past you guys.

There are three 'rooms' above the bridges: the trigger chamber, which starts the trap; the water tank, which contains the water used for dumping and whose water resets the trap; and the main reservoir, which feeds the water tank and the drowning area (not described here).

The first goblin (or any hostile) on the bridge passes over the bridge plate, which briefly sends power to the trigger chamber inlet pump. This dumps a small amount of water into the trigger chamber, which contains two pressure plates and the trigger chamber outlet pump. The first pressure plate is active between 0 and 1, and controls the water tank inlet floodgate. The second pressure plate is active between 2 and 7, and controls the water tank outlet hatch and the bridges. The trigger chamber outlet pump is currently unpowered. The water in the trigger chamber deactivates the water tank inlet floodgate, and opens the water tank outlet hatch and retracts the bridges.

Any goblins on the bridges fall into the pit below, and any goblins on the pressure plate get swept off into the pit by a torrent of water.

On the bottom level of the water tank is a final pressure plate, which deactivates once the water level drops to 1/7. This starts the trigger chamber outlet pump, emptying the trigger chamber into the water tank and resetting the trigger chamber's pressure plates. The water tank outlet hatch closes, the bridges extend, and the water tank inlet floodgate opens. The water level inside the tank rises, reactivating the reset plate, and deactivating the trigger chamber outlet pump.

Everything mechanical is now as it originally was, and the water tank finishes refilling as the goblins move across the first part of the bridge.

Below is a quick pic of the whole assembly that I've been using to check everything works. It might or might not be a help, but no harm throwing it in.

(side view)

Spoiler (click to show/hide)

The pic shows only the absolute nuts and bolts, especially with the screw pumps.

[Magua]

I need to make sure I have this right:

1) Pressure plate activates water inflow.

2) Water in the "trigger chamber" opens the water tank, and retracts the bridge.

3) Water spews from tank until it's near empty, which pumps water out of the trigger chamber, which then extends the bridge and refills the tank.

Yes?

It should work, although I can foresee problems if you get too many goblins coming through and setting off the initial reaction too quickly.  But it seems sound enough.

[Puck]

The framerate on my current fortress is slow enough

I wouldnt bother you with my simple design, but that caught my attention.

If your framerate is already low, why bother with moving fluids? (Although that design IS cool)

I got a bridge defense set up, that catches most invaders and shouldnt have an impact on the framerate Might wanna try it - keep in mind, this was explained to somebody completely new to DF 

[Sutremaine]

I need to make sure I have this right:

....

It should work, although I can foresee problems if you get too many goblins coming through and setting off the initial reaction too quickly.

Yep, that's how it works, but I'm not seeing the problem with the number of goblins. Ten touches of the pressure plate should have the same effect as one, and the pressure plate itself will be set past two full-length bridges.

If your framerate is already low, why bother with moving fluids? (Although that design IS cool)

Thanks.

The reason I'm using my established fortress is because setting up a new one would be too much fiddling about at my end of the keyboard, and because I went way overboard with the fluid systems when I first started. The excess magma is almost drained and the river has just been dammed, and that should give me some fps to spend on this.

I got a bridge defense set up, that catches most invaders and shouldnt have an impact on the framerate Might wanna try it - keep in mind, this was explained to somebody completely new to DF 

Ah yeah, I read that one. That's what gave me the idea to do my own version that deals with the stray runners you mention.