(now the Church of St. George]] in Thessaloniki built as a mausoleum for Galerius. The huge funerary hall or Constantinian basilica gradually fell out of use and into ruins, with the base of the wall now surviving for about a third of the original circuit of exterior walls, but Santa Costanza has survived all but intact. It is documented that Pope Nicholas I celebrated mass there in 865, the first time that "Santa Costanza" is recorded as its name, but its consecration as a church was not until 1254, by Pope Alexander IV, who had what were believed to be the remains of Constantia removed from the larger sarcophagus and placed under a central altar. Architecture Structure Santa Costanza is a circular, centralized structure, with a circular ambulatory ringing a high central space topped by a shallow dome, which is raised on a round drum, as can be seen from the exterior. It is built of brick-faced concrete and its structure is basically two rings supported by columns placed around a vertical central axis. The upper ring sits on the columns while the "lower ring encloses a circular ambulatory whose space flows between the columns into the axial cylinder." This design essentially creates two spaces or two worlds, that of the ambulatory and that of the upper dome. The screens of the ambulatory and inner ring create a dark contrast to the bright upper space of the dome. This contrast of light can be seen in the picture of the main interior. The single door, flanked by two arched niches, would originally have been an internal arch or doorway leading straight into the Constaninian basilica or funerary hall, half-way along its length. There is a short vestibule inside the door, opening to the ambulatory. An arched arcade with twelve pairs of granite columns decorated with composite capitals supports the drum below the dome, and separates the area of the ambulatory beyond, which is much darker, as light from twelve windows in the clerestory does not reach this area as well. In contrast, the central area is well-lit, creating interplay between dark and light in the interior. The number of arches, pairs of columns and windows could be a reference to the Twelve Apostles. Opposite the entrance in this central space there is "a kind of baldacchino...rises above a porphyry plaque which, below the middle arch of the center room, once