Watchtowers can have their doors toggled on or off. When off, you can't garrison units in the tower. When on, you can garrison units but one of the 8 sides will not be able to connect to a wall (as this would block the door).

 

TIP: You want to make sure your doors face the inside of your walls, if the enemy has access to a door in a Watch Tower they will use it to climb onto the walls and kill your troops garrisoned there.