Rotunda is a pericentral construction, unique in Greece. It was part of the palace complex built by Cesar Galerius in the first years of the Roman tetrarchy, when Galerius was based in Thessaloniki. One theory has it that it was built as Galerius’ mausoleum, but was never used, and during the reign of the Byzantine Emperor Theodosius the Great (379-395 AD) it was converted into a Christian church. It was then that a rectangular area with a semicircular apse was added on the eastern side in order to serve as the sanctuary of the church. Rotunda, approximately 30 m in height, is covered with a semicircular dome. The 6-metre-thick walls are built with stonework and strong ceramic mortar. At regular intervals, there are sections with bricks that reinforce the stonework. After the conversion of the Roman building into a Christian church, the monuments’s dome was decorated with exceptional mosaics. The variety of compositions, the multiple colours of the tesserae, and the great skill of the artists give an impressive result, where the divine and the transcendental are connected with matter and life on earth. The arches of the big conches are decorated with exquisite mosaics. In the panels birds, baskets with fruits, vases with flowers are spread over a gold or silver plain. The bright colours of the tessarae- green, light blue and red- give special splendour to the décor. In the conch of the sanctuary we can see the extant wall-painting depicting the Ascension of Christ, dated to the 9th century. The depiction develops in two bands. The upper band represents Christ on a mandorla held by angels, and the lower band depicts Virgin Mary in prayer surrounded by two angels and the twelve Apostles.