Minecraft wiki entry for Survival_in_an_infinite_desert, associated text: Bear in mind this ditch system does not (yet) stop spiders, so don't wander out at night and be careful coming out in the morning (particularly for creepers). Adding some cacti in the zone between the ditch and the house can stop spiders or at least harm them and alert you if they are there. If you have time, put a few cacti on your roof for the same reason.