Similarly to "the eyes being the windows of the soul," doors are the portals of the house through which one experiences a change of landscape, an insight into the beyond. Not exactly an "Alice in Wonderland" experience, but a transformative one, where the homeowner sighs with relief that they have finally arrived - home! Doors, whether entry or interior, create this passage, where the individual unhinges from his daily toils and enters another world, hopefully one of comfort and sometimes of surprise. That, I suppose, is the "Wonderland" feature of doors; we are never quite sure what lies on the other side. Doors create the "aha" moment for the perceiver, the rites de passage for the bride and groom, who symbolically cross over into another world, the unexpected. And, more often than not, doors are the happy symbol of a welcome home and the life therein it represents.
As an outsider, doors are very symbolic for what lies beyond. It is a preview, if you will, of the style and métier of furniture, furnishings, architecture, and way of life within. Doors define spaces; they separate and distinguish one from another. They divide the exterior from its interior. In this sense, they function as moveable walls which are hinged, tracked, concealed, divided, and applied with as many types of materials as the imagination can divine. But here's the rub. They just don't function as portals from one vista to another; they are themselves symbolic of what lies beyond and within. Their aesthetics speak volumes. Here form comes into play. So, when one sees a concrete door, one perceives resistance, an unwelcome entry to probably an unfeeling, cold interior. It is foreboding; it says "enter at your own risk." Conversely, a glass door parlays a feeling of lightness and transparency. Psychological barriers are broken down and a feeling of "peek inside and enter if you please" presides. But lest this observation seem too obvious and simplistic, think again. What we live in is representative of ourselves, doors included. That having said, these windows to the soul of the house can be broken down by purpose, material, and, and finally by style.
In terms of purpose, there are exterior doors, separating the outside landscape from the interior of the house. Then, there are interior doors. These are more malleable, separating room from room, room from its contiguous space. Sometimes, doors are freestanding like screens, serving the purpose of closure and privacy, though they are very open in concept. And, lastly, there are the little doors, the smaller size covers that preclude the viewer from seeing the contents of a cabinet. While more diminutive, they still say, "We're covering from view whatever is inside for a reason." And, then there are doors, like those in the Winchester house which lead to nowhere. Or, there are the famous "Beelzebub" doors, that only a Tim Burton can imagine existing, suggesting some portent of change, which brings one into another time and place. Doors are generally used however for privacy, security, and safety.