From Wikipedia: https://en.wikipedia.org/wiki/Vestibule,_Split

Built up to the beginning of the 4th century, as the original part of the ancient palace. It is a circular hall, once topped with a dome, 17 meters in height and 12 meters in diameter. Built as a grand meeting hall only for and selected audiences such as ambassadors. The entrance was adorned with a large door dimension of 2.56 x 3.96 ms with an abundance of relief decorations. The Vestibula area was cleared of four semicircular niches that were filled with statues of unknown deities. Southeast of the vestibule is the Medieval quarter, where the oldest building is the early Romanesque house from the 10th century. On the opposite, inside the church of St. Andrija (now part of the Ethnographic museum).