A semicircular place, as a semicircular arena, or room, or part of a room. "The collections will be displayed in the hemicycle of the central pavilion."
... with his usual assurance. And in the corridor that surrounded the hemicycle they climbed up on to a narrow ledge in the wall and sat side by side in perfect luxury, not dreaming that they were doing anything unusual or undignified. As a fact, they were not. Other couples were perched on other ledges, and ... — The Roll-Call • Arnold Bennett