Stadium

The Piazza Navona‘s cloistered shape (round at the top, squared off at the bottom) is because it was built atop the ruins of the Stadium of Domitian.
The stadium was used almost exclusively for ludi (sporting events like what we think of as Olympic track & field.) 
  
“Vomitorium” isn’t what it sounds like.  It’s actually a passageway through a tier of seats.  The piazza’s buildings incorporate what was the stadiums lower arcade.