Here is a design I came up with to automatically catch just about anything in the caverns
A goal I have in any fort is to catch as many cavern creatures as I can and train/breed them (cave crocs at the top of the list). But building and watching over cage traps for three different levels of caverns, plus make them all resistant to FBs was troubling. So I took a series of tunnels I had come up with to manually catch Titans and Forgotten Beasts, and automated it.
#1 This first image is the top level of the trap
(http://i.imgur.com/5ZThDEb.jpg)
Levers to control the filling of the water reservoir, as well close off the individual cells on the left. When closed, each cell has plenty of water for use in the trap.
#2 Second image shows the main level of the trap.
(http://i.imgur.com/uOMCA3G.jpg)
Cage traps line the front, then a ladder-like pattern of tunnels. A floodgate is located in each path across, directly underneath the hole in the ceiling to the water reservoir. A pair of doors lie next to the floodgate, to give the building destroyer something to munch on while the trap activates.
The levers on the right correspond to the same path at its level, and are linked to the two drawbridges to release their occupant (or close the path) at your discretion.
Also note the GCS and pig in the lower right. Once a web FB or a GCS is caught and a web trap setup, the other FB caught by the automated trap (not web FB) may be released and secured in simple wooden cages for display in your dining room.
#3 Bottom level of the trap
(http://i.imgur.com/yQPqjTx.jpg)
This level is nothing but closed off pressure plates set to 1-7 water, reusable, and linked to the two drawbridges for that path on the main level.
How it works:
FB spawns, paths to the entrance to the trap on each cavern level. Bypasses the cage traps, runs to the nearest unused cell, and begins to destroy the floodgate.
Once the floodgate is gone, it moves on to the doors; meanwhile, the cell of water above the floodgate is released, rushes down, and will pool in the lower cell with the pressure plate, triggering the trap. By the time the first door is gone, both drawbridges are raised and the FB is trapped.
(http://i.imgur.com/TnJTzNO.gif)
Some tips:
-If you have Gremlins in your caverns, keep those levers walled up until you need them! Also possibly station a military squad to train next to them.
-Floor over some of the trap area; it will get wet, and the occupant may be walled up for quite some time. Tree growth may inadvertently lock them away for good without woodcutter intervention.
-Use an aquifer or a reservoir to fill the trap. Cavern water is decent, until it gets low enough to allow the growth of trees held stagnant. As you can see in the top picture, One such tree popped up when I first filled the trap, and almost blocked the intake.
-Be careful once you set up the web cage trap; merchants loved to exit that way in my fort, and would walk right into it.
-Never release a web FB unless you know what you are going to do with it! They cannot be caged by a web cage trap, and are thus best kept contained or positioned (using doors and drawbridges) into a silk farm or a web turret for sieges.
-Use of the ladder design, without any automation, is a great early-fort defense against Titans and Forgotten Beasts. Each 'cell' requires 2 blocks for the bridges, 5 mechanisms to link the drawbridges to a single lever, and 2+ doors (more = more time to pull the lever!), while the trap itself should be closeable as well, so another block for a drawbridges, and 3 mechanisms to close it up.
Here's a link to the imgur gallery:
http://imgur.com/a/ucMAG#0 (http://imgur.com/a/ucMAG#0)