Normal doors are barriers that open automatically when the ninja is nearby. Their primary function is to keep a drone confined to an area or to prevent the ninja from being attacked by rockets, gauss turrets, or a drone's ranged attacks. Unfortunately for you, they do not block floorguards and thwumps.
If the ninja moves into a space (the square area occupied by a tile) that is adjacent to the normal door, the door opens. While the door is open, enemies and projectiles can go through it. When the ninja moves away again, the door closes. Because a door opens any time the ninja comes near it, they rarely obstruct or hinder the ninja's movement significantly.
If you place 6 pieces of gold on a normal door, the ninja will bounce off when it hits it. This is useful for propelling the ninja. Also, if you place 12 pieces, you can use the door twice; 18, three times, etc. This is a technique used numerous times in DDAs.