You enter a long, high, dark passageway formed by tufa block walls and concrete barrel vaults (the image shown here is actually a corridor in the amphitheater at El Djem, Tunisia). As you enter, you look from side to side at the "crowd movers" -- the corridors (ground floor of the Colosseum) and ramps beneath the cavea which allow people to quickly enter and exit from different segments of the general seating (floorplan of the Theater of Marcellus).
Even though you will not be permitted to sit there, you walk straight ahead, taking the main passage directly into the orchestra area; your view resembles this one, which shows the central vomitorium of the Theater at Ostia.
Orchestra of the Theater of Marcellus