A monopteros is a circular colonnade supporting a roof but without any walls. Unlike a tholos (in its wider sense as a circular building), it does not have a cella. In Greek and especially Roman antiquity, the term could also be used for a tholos. In ancient times monopteroi served inter alia as a form of baldachin for an idol. An example of this is the Monument of Lysicrates in Athens, albeit the spaces between the columns were walled in, even in ancient times.