Monopteros

A monopteros is a circular colonnade supporting a roof but without any walls.

A monopteros is a circular colonnade that supports a roof but encloses no walls or interior chamber, distinguishing it from a tholos, which is fully walled. The name comes from the Ancient Greek for alone and wing, and in Greek and Roman practice a monopteros sheltered a cult image, functioning much like a baldachin; the Monument of Lysicrates in Athens and the Temple of Roma and Augustus on the Athenian Acropolis are ancient examples. The form later became a fashionable garden feature in English and French landscape gardens, often built over a well in a park or spa. Most monopteroi are circular rotundas, though some take square or polygonal plans, such as the hexagonal Muses' Temple at Tiefurt House.