propylaeum, in Greek architecture, a monumental entrance to a sacred enclosure, group of buildings, or citadel. A roofed passage terminated by a row of columns at each end formed the usual type. Known examples include those at Athens, Olympia, Eleusis, and Priene. The most splendid example are the Propylaea at Athens upon the west end of the Acropolis; their restored remains still stand. Of Pentelic marble, they were built (437-432 B.C.) at the command of Pericles by the architect Mnesicles.