Marble Wall

Marble caves come with their own background wall, and that version does not seal anything. Hoplites and Medusas keep appearing inside a fully enclosed pocket of it. Only the wall inside a generated cabin, or wall you place from your own stock, counts as covered ground.

Knocking the natural wall down returns nothing to your inventory, so anyone turning a marble cave into a base or an arena has to haul crafted wall in with them. Do the entire pocket in one pass. A half-covered room still reads as open to the spawn code, and you will end up fighting in your own kitchen.