"The parodos gateway and its curtain."
Cf. W.E.B. Dinsmoor, The Architecture of Ancient Greece (London 1950) 318-319:
"In some of the Hellenistic theatres the parodoi assumed greater regularity, enframed between the oblique retaining walls of the auditorium and the approximately parallel terraces of the ramps to the proscenium. An organic connection was established in some cases by the insertion of parodos gateways, likewise oblique and at no logical angle to anything else, as at Priene, Delos, and Epidaurus; in the last they are double, the smaller opening giving access to the ramp. And in a few instances such gateways are placed in more orderly fashion at right angles to the scene building, as at Assos, Babylon, and Segesta; likewise at Pergamum were removable wooden parodos gateways at right angles."
Yours is the theatre at Pergamum, n'est-ce pas, Euterpe?