At the southern end of the peristil, above the basement stairs, is the vestibule, a grand and cavernous domed room, open to the sky, which was once the formal entrance to the imperial apartments. If you're lucky, you might come across a klapa group here, taking advantage of the acoustics for an a cappella performance. Beyond the vestibule and curving around behind the cathedral are the ruins of various Roman structures, including the imperial dining hall and a bathhouse.