The Church of San Vitale is a centrally planned church that has a nave that surrounds the central space. The columns inside the church are doubled and made of high-quality marble that was brought from the east. These columns left the classical orders behind and instead tried to invent a new iconography for their architecture. The early Christians did this by making impost blocks that help the transitions up to the arches. The eastern end of the San Vitale is an extension with an apse at the end. The apse-like shapes are supported by columns and piers that also support the building. The apse is covered in a dense mosaic (tiny pieces of glass, gold, and other precious metals [rubies and emeralds]). The mosaic reflects light through its large windows. The walls are covered with windows and are used to reflect the light of the mosaics. The outside of the San Vitale is octagonal with a smaller octagon that rises higher (dome-like). The exterior is made of brick taken from the ancient Roman buildings and reused to make the church. The walls are covered with large windows that are used to reflect light through the mosaics. There are a variety of panels and artworks displayed all over the church that are represented as mosaics.
As you can see, this church is an octagon. Yes, an octagon. Not a square. Not a circle. This church is an octagon. Possibly a dodecahedron, but most likely just an octagon.