Minecraft wiki entry for Things_not_to_do, associated text: On Hard difficulty, zombies can break down wooden doors and attack the player while they are within their base. There are multiple ways to protect a wooden door from zombies in hard difficulty such as placing a block front of the door anytime they are not using it, using an alternative door (fence gates, iron doors, trapdoors, etc.), placing a slab at the top of the front of the door, or placing a door from inside the door-hole so it is opened to close off the door opening. Another way to block zombies from doors is to place the door sideways, so when the door is open it closes off the doorway. The zombie's A.I. treats it as open and prevents the zombie from attempting to break it (instead of trying to walk through it). Tutorials/Traps has more ways to protect the player's door from zombies along with other threats such as players.