Minecraft wiki entry for Defense, associated text: Dry ditches, like walls, should be at least three blocks tall on the inside edge, with an overhang or protrusions to prevent spiders from climbing them. They should also be at least three blocks wide at their narrowest point in order to prevent spiders from jumping over them. It is recommended that players leave a means of escaping the ditch to the outside (not the inside, which would defeat the purpose of a ditch), in the event that they are knocked into it by injuries, explosions or simple mistakes.