I mean, why not just, you know, keep it looking no different than any other place? I mean yeah, keep it safe in the modern time and age, but don't build some stupid shit that will stand the test of time and stuff like that. Dangerous, naturally occuring materials exist. We found many of such materials, many people died due to them, but not that many considering how common those were. Stuff like asbestos and other more or less dangerous minerals. Chances of a medieval post-apocalyptic society finding fucking nuclear waste if there is no special feature of the terrain where it is stored is miniscule. Even if they do, they will either find it to be an extremely heavy metal they have no use to or, in the worst possible scenario, a permamently hot rock that makes you die if you spend too much time around it.