The megaron was the great hall in very early Mycenean and ancient Greek palace complexes. Architecturally, it was a rectangular hall that was surrounded by four columns, fronted by an open, two-columned portico, and had a central, open hearth that vented though an oculus in the roof.
Foundation of the megaron complex at Mycenae, view from the main hall (circular hearth visible in foreground) through the anteroom and porch