Minecraft wiki entry for Building_a_metropolis, associated text: Defense Don't let the monsters take over the city! Make sure you have walls around your metropolis. You can put cacti on top of the wall to keep spiders out. You can also add archer towers if you want. Just make a 3×3 square on the wall with ladders to get up to them. Add moats with lava for the death of your enemies. Just be creative!