It is located on the northern end of the Accademia Esplanade. Only the lower floor is still standing, a massive cube of masonry with three main façades preceded by a porch; the holes for the beams can be seen on the walls, and its foundations are visible on the ground. The fourth side on south-east is adjoining the retaining wall of the Accademia Esplanade (plan fig. 22, plastic model fig. 23, aerial picture fig. 24).
Near the building is a masonry ramp supported by a series of arches, leading up to the Accademia Esplanade.
The main entrance is located on the north-western side of the lower floor1, and gives access to a large circular hall (RB6) with a diameter of 9,50 meters. It is covered by a dome of the same height, which means that the room was designed around an imaginary perfect sphere, as happened in the Pantheon of Rome2.
The circular hall (figure 25) has rectangular and semicircular alternating niches; in front of the entrance door is a niche with an apse for a statue. In the dome, above the rectangular niches, there are four small slots3 which are the starting inner point of five conduits that pass through the entire thickness of the masonry and come out on the four sides (figure 26).
The first three conduits A-B-C (plan figure 27) open at the center and on top of the three main façades of the building, facing north-west, north-east and south-west. The other two conduits D-E are on the south-eastern side, and open at the two sides of the staircase that once led to the temple.
From one of the rectangular niches of hall RB6 (see plan fig. 22) a corridor (RB7) led to a circular latrine (RB8); another symmetrical latrine is on the opposite side of the building (RB8bis), where the Jesuits built - without knowing it - a small chapel. Corridor RB9 gives access to substructive rooms RB10-11, which are supporting the staircase of the upper floor once leading to the temple on top.
The upper floor has been completely destroyed and presently is a wonderful panoramic terrace. Originally there was a circular temple, which was reconstructed by Lugli and Bonelli4, studying the marble architectural fragments that are still visible scattered on the grass around the building (figure 28 and plan figure 33).
Fifteen out of sixteen travertine blocks which supported the bases of the columns are still in place; the 16 columns of doric order were made of white marble and had a diameter of 0,70-0,80 meters; their estimated height was about 6,5 meters (22 roman feet).
The inner cell of the temple was octagonal: in its four larger sides opened the main door and three windows; the shorter sides had rectangular inner niches (see plan figure 33).