The first level of the Basilica of Justinian - this was an absolutely enormous combination of chamber of commerce and law court - basically all the high end business deals and legal matters got dealt with here.  The tops of the arches are probably 150-200' up.  That big chunk of ceiling there in the foreground was probably 30' high and this was only the first level - domes rose up from here to cover pretty much all of the south end of the Forum