Minecraft wiki entry for Defense, associated text: Cobweb ditches are more compact than regular ones. They are one block deep and one block wide ditches filled with cobwebs. Mobs get stuck in the cobwebs, immobilizing them to be killed. They can be easily jumped over. This ditch is useful for making a border for your safe area, the outside of which does not need lit up. Cobwebs are found in mineshafts, stronghold library, spider spawner rooms of Woodland Mansions, and zombie villages. Note: spiders and cave spiders can move freely in cobwebs without being stuck, also some mobs capable of ranged attack can still attack player even though they are stuck in a cobweb.