I presume this to be the altar with four columns surrounding it. A viewer let me know "Correct this is the altar. The four columns would have supported either four lintels, or a canopy. This would be called a ciborium or baldicchino, and could have curtains hung between the pillars. My guess is that some of the pieces in the courtyard came from this structure, and the two pillars in front of these. "

A fine example of a marble ‘opus sectile’ mosaic pavement. Byzantine, 11th century.

Opus sectile is an art technique popularized in the ancient Roman and medieval world where materials were cut and inlaid into walls and floors to make a picture or pattern. Common materials were marble, mother-of-pearl, and glass. The materials were cut in thin pieces, polished, then trimmed further according to a chosen pattern. Unlike tessellated mosaic techniques, where the placement of very small uniformly sized pieces forms a picture, opus sectile pieces are much larger and can be shaped to define large parts of the design.

Correspondent: J.M.Criel, Antwerpen.
Source: Website of ‘stnicholascenter.org’ & Wikipedia.