Granite Wall

The wall a granite cave generates does nothing to stop enemies appearing, which is why those caves stay busy no matter how enclosed a pocket looks. The identical wall inside a generated cabin does block spawns, and so does any of it you place yourself. Same background, opposite behavior.

Hammering the natural wall gives you nothing back, so every piece you build with has to be crafted stock you carried in. Plan for that before converting a cave into a base or a farm. Clear the natural wall and re-cover the whole pocket in one pass, or the room you thought you sealed keeps spawning things behind you.