Boobytraps. Rumours about a curse/ghosts/entry fee. Pretend there's something boring in there. Allow entry, but make the process the full force of German bureaucracy (you need to file form A on a blood moon, to get form A, you need document B2, no not that one the green one, you can get it from Location C but that's only open once a month for half an hour, and if you don't wear pink they won't help you).