Wood Wall

The point of this block is not how it looks. Any wall a player places blocks enemies from appearing behind it, while the natural background you see in caves does not. That is why a plain wooden box is safe to stand in and an open cave alcove is not.

It is the cheapest way to seal an arena, and it is what makes a room count as a house for a town NPC. Wood is abundant from the first minute of a world, so there is no reason to be stingy. Back every corridor and every platform level you plan to fight on.