Apsidal wall comprised of multiple phases, cut by drainage channel and running under W balk. Wall potentially also runs beneath S. balk. Potentially associated with similarly curved wall (SU 2219) in the N. of area. 3 cut stone blocks present, but the rest of the wall is composed of irregular stones of local tufo with occasional travertine and basalt. Delimits the compound containing Rooms 1 and 2.