I found "The lower terrace (100 x 50 m) was built first in 27/28 AD by "Diodoros, son of Zebedas, architect from Gerasa". A unique feature is a vaulted corridor that ran around the periphery, and which was embellished with facades of Ionic half-columns supporting a Doric frieze. In 162/163 AD, a large temple was built on the higher ground above the sanctuary and a grand staircase was added into the west facade of the lower terrace to access it." This may be part of that vaulted corridor, but I'm not certain.