The description of a Door in encyclopeadias is so simple that there shouldn't be looked for an other meaning behind it. A Door, is a barrier made of different materials like, wood, stone, metal, glass or a combination of metarials, installed to swing in order to close an opening to a room or building. Doors have been symbolic across cultures for as long as history has been recorded. A door is both an entrance and an exit, so it has been united with portals and passageways.
Doors were first seen in history on paintings inside Egyptian tombs but the physicaly used were made of hides or textiles used in Mesopotamia and the ancient world. Simply to cover or hide the inner ambience of the private room. Even these were made of stiched colorful ornaments. Doors which were made of rigid metarials like, stone or bronze appeared with monumental architecture. With the beginning of the Medieval Age, the structure and the important of doors, gates or passages changed.
Doors have always been used for expressions through the medieval ages. Expressions placed at the entrances of buildings gave messages of the owner of the building, or to be aware of what will or can happen after passing the door, or illustrating a historical scene. Therefore, because of its importance, it can be seen that the doors of, in western medieval ages, religious and non-religious buildings are masterpieces of the on its own.
With the modernization and cost cuttings in architecture, the simplicity changed the importance of doors. It just became a part of a building. Therefore, we are always amazed seeing the beautiful doors of the medieval age's constructions.
There is also the spiritual meaning where doors signify communication and conjunction in religious books.
He commanded the skies from above; He set open the doors of the heavens (Ps. 78:23);
I have chosen to stand at the door in the house of my God, rather than dwell in the tents of wickedness (Ps. 84:10);
Behold I have set before thee an open door, which no one can shut; for thou hast a little power, and hast kept My word (Rev. 3:8);