There's three ways to capture a creature in a cage. 1) Let it walk over. Most creatures work just fine this way. If it is [TRAP_AVOID] then you need to get more creative. 2) Knock it unconscious. All traps work on all creatures which are unconscious, including your own dwarves. You can kill/cage your own men by putting them in a roomful of weapon/cage traps and waiting until they sleep. For invaders, this usually means getting lucky with a stun from a weapon, a stun from a cave-in, or passing out from pain. For all intents, "stunned" means "unconscious". 3) Place a Giant Cave Spider web atop a cage trap. I can go into detail (in fact I think I already made a thread about it) but the basic GCS silk farm setup involves a walkway with a wall on one side and nothing on the other. Both sides have hatches covering empty space, as the only way out, and a pressure plate beside each hatch that triggers said hatch. This way, when a creature is inside the farming rig, they cannot escape, because every time they walk to the exit they hit the plate and the exit disappears. You then place some bait across the way, which is why one wall of the walkway is missing, and the GCS will fire web at the bait animal so long as it can path to it. Using this, the GCS will see the creature, and can path to it, so it will fire web at it while running. It hits the plate, its path disappears, and it cannot escape. It then turns around, taking the other path while also firing web. Once it reaches the other side, the first has already re-opened. Utilize a drawbridge to selectively block view, which will disable the farm and allow your dwarves to collect silk. Build this at your entrance, and litter it with cage traps, and you'll have the spider webbing all the cages for you. Using this, I managed to collect a few dozen FB/Titans one time.