The entrance to houses in ancient Rome was called the ostium. The term ostium includes the doorway and the door itself, and the word is applied to either one, although fores and ianua are the more specific words for doors.
In poor houses the ostium was directly onto the street, and it seems certain that it originally opened directly into the atrium; that is, the ancient atrium was separated from the street only by its own wall. The refinement of later Roman times led to the introduction of a hall or passageway between the vestibulum and the atrium, and the ostium opened into this hall and gradually gave its name to this hallway or assembly hall. The door was placed far back, leaving a broad threshold (or "limen"), which often had the word "Salve" inscribed on it in mosaic form. Sometimes over the door were superstitious words of good omen, or a charm against fire.
In the Roman houses where an ostiarius or ianitor was kept on duty, ie a caretaker, his place was behind the door; sometimes he had a small room.
Dog Wall Picture.
A dog was often kept chained inside the ostium, or in place of one a picture of a dog was painted on the wall or worked in mosaic on the floor with the warning beneath it: Cave canem! (Beware of the dog!) The hallway was closed on the side of the atrium with a curtain (velum). This hallway permitted persons in the atrium to see passersby on the street.