Complex sentence: The oldest surviving parts of the church building date from at least the 4th or 5th century; these include some pieces of floor mosaics, and traces of frescoes on the right side of the altar.
Split into simpler sentences: The oldest surviving parts of the church building date from at least the 4th or 5th century. These include some pieces of floor mosaics, and traces of frescoes on the right side of the altar.