A small straight forward structure that I like to use. Jumping directly over the gap to reach the door will normally cause enemies to get hit and fall down into the pit (and get hit again). The way to get in is to first drop down into the put (on the trap bridge) and then jump upwards into the door. I caught a lot of enemies in this, since they seem to be unable to comprehend that they cannot touch the door without entering certain death. Easy to build and a perfect iteration to a Linepiece.