Minecraft wiki entry for Shelters, associated text: If you haven't slept in 3 or more in-game days, beware of phantoms, which can reach you on a pillar. Otherwise, watch out for climbing spiders or even (unlikely) a spider jockey. To fend off spiders, you can break one of the blocks below your top block, or build a lip around the block you're standing on. You do either of these by crouching as above, and placing or breaking blocks. You will not fall unless you let go of the shift key while leaning over the edge… or unless you are attacked, so don't do this if a spider is actually getting close to you (or if your tower is under 20 blocks and a skeleton is at the base). If a spider does climb the pillar, they're fairly unlikely to actually reach you, but, just in case, keep your stone sword in hand and whack them as soon as they come into reach, before they get up to you. (Attacking them will knock them down, causing them to take damage from the fall.)