Fantasy idea; a Mordor like valley nestled among mountains is so defensible that it hasn't been the home of one BBEG in the past - it's been the base of several. The surrounding countries have it blocked off by treaty so nobody can really build an army there, but a few adventurers or bandits aren't an army and usually get in over the mountains. It's sort of magic Pripyat, returning to the wild and full of treasure, but also lethal magic energy, from the last few wars.