Minecraft wiki entry for Shelters, associated text: Going 10 or 12 blocks up will usually be enough, 16 is safer (skeleton range), and 20 or 30 is more certain. Zombies can track a player 30 blocks up, but they cannot reach the player. You can also use sneaking (holding ⇧ Shift) to lean over the edge to place extra blocks as a ledge. Then you can put your crafting table (and soon, your furnace) on the ledge and work overnight. Remember to retrieve them before you come down! You can look around and see what's happening overnight, but try to avoid putting your crosshairs on an enderman.