Complex sentence: The rotunda was built in this same fashion, travertine block on the outermost section with cement poured in the middle to give the concrete some structure and covered in Travertine revetment, must of which has been stripped away.
Split into simpler sentences: The rotunda was built in this same fashion, travertine block on the outermost section with cement poured in the middle to give the concrete some structure. The core of the building was mainly built of concrete and covered in Travertine revetment, must of which has been stripped away.