The mobs (zombies, skeletons, animals, etc.) can't build or destroy blocks.
Did you make sure to light the area appropriately with torches or something else? Hostile mobs like zombies, skeletons and spiders can spawn (spontaneously appear) anywhere that's dark enough while you're away. Alternatively, you have a 1x1x2 hole somewhere for the zombie to have walked in.
Did you make sure to light the area appropriately with torches or something else? Hostile mobs like zombies, skeletons and spiders can spawn (spontaneously appear) anywhere that's dark enough while you're away. Alternatively, you have a 1x1x2 hole somewhere for the zombie to have walked in.