n.
In ancient Greek architecture, a structure forming an entrance or gateway to a sacred enclosure, usually consisting, at the least, of a porch supported by columns both outside and within the actual gate.
The term is often used in the plural (propylaea). The most famous example is the great Propylaea designed by Mnesicles for the Athenian Acropolis . The name propylaea was also applied to various 18th19th-century Neoclassical and Romantic monumental gateways.