Minecraft wiki entry for Defense, associated text: A clever way to fight spiders is to dig a one wide two deep ditch. Spiders cannot pass through it, but players and another mobs that are 1x2, can. This way, you can simply hit the underbelly of the spider. Spiders also can climb walls, but adding an outcropping or overhang prevents them from getting over since they can't wall-climb on ceilings.