I know - to make sure that everyone gets equally annoyed by this, let's just ban invis!
[sup](this was not a serious suggestion maybe, please don't kill me)[/sup]
Alternatively, either the requirement for a way out works, or it's the raider's fault and they should be left for dead, perhaps with some way to suicide I think. They take the risk to go through the vault fully knowing they could get trapped in - they see that second dirt layer, so they know it's going to be risky, and they have the option not to go in. You could say there needs to be that risk. I disagree that it's "punishing raiders" by leaving them stranded - they've got way more of an advantage by being relatively undetectable by the raided, and they know it's risky. You could say it discourages raiding, but invisible people discourage defending against raiding, and that will lose you players. Invisible people aren't fun to deal with when they're within the walls, and that's why we've got this massive debate here. At least banning invis solves all this. :S