Minecraft wiki entry for Shelters, associated text: To block the cave off, for your first night you can use walls of dirt or cobblestone similarly to the "hole in the wall". If you have enough wood, you may be able to craft fences and a fence gate and place them across the entrance and any openings in back. Make sure you place the walls or fences behind the upper lip of the cave (or extend the ceiling over the barrier), or monsters are liable to "drop in" inside your barrier. Also, remember to light the area before you settle down. Likewise, remove any stray blocks, within two spaces outside the fence, from which monster could jump onto the fence (try the jump yourself). If using fences, also make sure you can retreat into the cave and away from the entrance (out of sight or 16 blocks away), or a creeper may drop by and just wait for you to come out. Later, you can be more sophisticated about sealing off and fortifying your lair.