Tutorial:Defense

Some of the best defense measures are those that prevent hostile mobs from spawning entirely or at least prevent hostile mobs from ever getting close to the protected area or defense mechanism. As such, integral to all Minecraft defense systems:

Passive defenses prevent hostile mobs from entering the protected area without any input or sensory devices. The most common example is walls: they do not have to sense a mob coming or sense its position to prevent it from entering a walled off area. The following sections discuss various types of passive defenses, their construction, maintenance, and applicability.

Passive defenses generally fall into one or more categories, sorted by their impact on an aggressive force.