Minecraft wiki entry for Defense, associated text: Place a dispenser facing upwards in the ground. Place a pressure plate on the outside edge of the dispenser. Put a flint and steel in the dispenser. Now, when something tries to walk past it, they will trigger the pressure plate. This will cause the dispenser to use the flint and steel, lighting the entity walking past it on fire. The fire will go out shortly, but it will cause massive damage to most mobs.