You can build bridges to crush invaders.
Two 2x6 bridges set to raise south (can be larger, 2x10, 3x10, etc), - is the bridge tiles when lowered, = is where the bridge raises to, P is a pressure plate linked to both bridges, S is a statue to make things walk over the plate.
Be sure the plate is set to not be permanent.
What this does: Enemies walk through hallway, one in front steps on pressure plate. Bridges raise and then immediately drop back down (since not permanent), crushing everything in the hallway - usually even the one that triggered it, since he takes another step forward before it triggers. You COULD also set the second bridge to raise to the right, flinging invaders as they walk across the pressure plate. I enjoy crushing them myself, though.
This can also be done with lava, just put a lava floodgate on the end of the hallway set to not permanent, stepping on the plate releases a flash flood of lava, burning up everything in the hall. Be sure to put safety channels to prevent a flash flood of the entire world (unless you want that?).
This won't work with water, since they won't drown in the short flash of water. It would be a great way to irrigate a room safely, but you can't get the pressure plates to trigger when you want them to. Maybe in the future we will be able to set levers to not permanent or reset on a timer.
[ March 28, 2007: Message edited by: Paul ]