Mudstone Brick Wall

Natural walls almost never block enemy spawns, which is why the standard move for a cave you want to live in is to strip the background out and put your own back up. The version already generated inside a Jungle Shrine is the exception. It counts as safe exactly as it stands, so you can clear the room, seal the way in and treat the shrine as housing without rewalling anything.

Not every world generates shrines built from it, and breaking a wall returns it as an item, so one shrine is usually all the material a build needs.