A house means a roof overhead and four walls, but the external door is the way in or out, and the house is not much good without it. External doors have to stand the elements and frequent opening and closing.
External doors are usually wood, but may include glass for windows and metal fastenings and locks. Finish can be just about anything: paint, varnish, natural hues or something more colourful. Enter your home sweet home via an external door.