Going to have to figure out how to prevent the door from being lifted after being triggered – then we’ve got ourselves a trap!

Easy enough. Swinging bar. Here, we use a cutoff broomstick to help keep windows shut against intruders. I wish I had an image to show you… would be easier.

Think of your wire cage door above the hallway. Trapped above the cage door is a steel rod laying on its side and connected to one corner by a pivot. When the cage door falls, gravity will cause the steel rod to swing down into an upright position, preventing the door from being slid upward. How to remove the steel rod and open the cage door when the cops arrive? Simple. A string tied to the free end of the rod and fed through a hole at the corner opposite the pivot will allow you to pull it back up.

Simple, elegant, effective. Almost no moving parts to fail. If power fails, the steel rod will still do its job, since nobody can turn off gravity. Well, at least not yet anyway…

The wicked flee when none pursueth..." - Proverbs 28:1