Doors are self-explanatory. You open them by pressing E. You can also break wooden doors down by hitting them with a weapon. If the door gets stuck, breaking them down is the only way to open them. They are located on the dungeon and ruins floors, and there are three types.
The wooden door is only located on the dungeon floor. It is, as the name suggests, made of wood. This is the only type of door that gets stuck, and it can be bypassed by breaking it down with a weapon.
The stone door is a lookalike of the stone walls located on the dungeon floor. It is most often found at the end of a hallway in order to simulate a dead-end, however it has also been found in rooms.
The locked door is a unique door located on the ruins floor. It is necessary to open it using the key located elsewhere on the ruins floor, and leads to the Yithidian Orb at the end of the dungeon.