The Vestibule is the circular domed antechamber of Diocletian's Palace — built in the early 4th century as the formal entrance to the emperor's private apartments, and now open to the Adriatic sky through an oculus that was never designed to be open. The dome lost its covering centuries ago; what remains is a perfectly proportioned stone cylinder with four arched niches, the oculus above admitting a column of light and, in summer, blue sky. The floor is the original Roman stone. The walls are alternating courses of brick and dressed limestone. The effect is complete and unexpected — a 1,700-year-old room in the middle of a working city, still performing its spatial function of transition and pause. It is among the more affecting architectural experiences available without a museum ticket.
Diocletian's Palace, Split, Croatia