<p>A circular complex on the *Esquiline outside the *Servian Wall, known from an inscription found near the *Porta Esquilina and Piazza Vittorio Emanuele II (<i>CIL</i> VI 29774: A DI(A)ETA APOLLINIS) which mentions two circular promenades (<i>gestationes</i>) with diameters of 124 m and 94 m. Häuber has matched these dimensions onto the archaeological remains S of the *“Via Labicana-Praenestina” known as the ‘hemicycle’ and, extending the hemicycle into a full circle, she discovered that this reconstructed second half perfectly reflected the topography of the valley in 1850 (Häuber 85-87). The roofing system shown on the map follows her reconstruction, based on an analysis of the circular walls that do survive (Häuber 91-92). Also shown is a long and narrow <i>euripus</i> (channel of water) which extended S from the outer circumference of the circular structure (Häuber 91, map 3 no. 15). The Diaeta Apollinis is dated to the Augustan period, since its <i>opus reticulatum</i> masonry is similar to that of the *“Auditorium Maecenatis” (Häuber 97). Its function is not known, but it was probably part of a luxury villa complex, either the *Horti Maecenatis (as legitimately argued though not proven by Häuber 97) or, following the equally valid traditional reading, the *Horti Lamiani (Grimal). Cima di Puolo continues to accept the traditional reconstruction of the remains as a hemicycle with portico; she suggests it was a monumental <i>nymphaeum</i>, which opened onto the valley to the W.</p>