Minecraft wiki entry for Shelters, associated text: Players (and mobs) entering your house without your knowledge is a pain, that's why you need a doorbell. When a mob or player steps on the pressure plate the note block will play a sound, and you can make this more sophisticated making chimes or even a whole song that play upon entering. If you have multiple entrances to your base, place different blocks under each note block to make sure that if a creeper comes to your west entrance, you don't get blown up when you try to run away through that door.