Imperial palace of the emperor Domitian (51-96 AD). It is still uncertain the precise function of this large building, also known as Domitian's stadium (Stadio di Domiziano) or "Circus Agonalis". The stadium was part of the imperial palace and was surrounded by a two-story portico. Maybe it was not just an hippodrome, but more in general, it was the “viridarium” (the garden), private place where the emperor he could walk on sunny days and watch the races.
This is the best conserved construction of all three pieces of the Palace of Domitian. It was also built according to a project of architect of Domitian, Ribarius. It served for the sport competitions, horse races and, probably, as a garden for the imperial family. The stadium is 146m long. The arena was surrounded by a two-storeyed portico with engaged columns covering a wide ambulatory or cloister. In the middle of the east wall is a wide exedra of two storeys, which served as an Imperial box.