The site with its sheltered harbor was first settled by Illyrian tribes in the Bronze and Iron Ages. Conquered by the Romans in 178 BCE, it was elevated to colonial rank, Colonia Julia Pola Pollentia Herculanea, between 42 and 31 BCE. The amphitheatre is the sixth largest surviving Roman arena. It was built near the sea to the northeast of the city. Seating approximately 23,000 spectators, it is enclosed by a wall (h. 32.5 m) with three storeys on the seaward side and two on the inland side, where the seating rested directly on the sloping ground. It is the only remaining Roman amphitheatre to have four side towers and with all three Roman architectural orders entirely preserved. It was completed in 81 CE under emperor Titus.