Minecraft wiki entry for Tips_and_tricks, associated text: Zombies can break down doors, if the game difficulty is set to hard when night falls. Here's a solution that doesn't require the player to craft an iron door or compromise the base's security. Simply break down the door, then turn so that the player is perpendicular to where the door just was, and place the door. Get out of the hole the door is occupying, and open it by right-clicking. Now, if the door is placed it the right way, zombies will think that the door is open and avoid it. One can also place any regular block such as dirt or cobblestone in front of the door to prevent zombies from getting to it. If the block is placed on the outside side of the door, the zombie will not be able to damage the door. If the block is placed on the inside side of the door, the zombie can break down the door, but cannot proceed further inside.