propylaeum, in ancient Greek architecture, porch or gatehouse at the entrance of a sacred enclosure, usually consisting of at least a porch supported by columns both without and within the actual gate. The most famous propylaeum is the one designed by Mnesicles as the great entrance hall of the Athenian Acropolis (begun in 437 bc).
The name propylaea is also applied to various monumental gateways, Neoclassical and Romantic in style, built in the late 18th and 19th centuries. They include the Propyläen of Munich (1862) and Berlin’s Brandenburg Gate (1784).