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).

Quick and dirty Google search says GDPR law is complicated when it comes to backups https://www.itgovernance.eu/blog/en/the-gdpr-how-the-right-to-be-forgotten-affects-backups-2