Minecraft wiki entry for Trap_design, associated text: Arrows can deal damage to players either by being propelled by TNT, dropped from a trapdoor, or most commonly, from a dispenser. Arrows from TNT cannot be reliably directed. Arrows dropped from a trapdoor can only cause minimal damage to the target. Dispensers can fire very rapidly, and far which makes them ideal for an enclosed space like a tunnel. The dispenser must be reloaded from time to time.