While touring in Caesarea, Israel, our guide Danny the Digger made a very interesting observation. We wandered through the ruins of the Roman city built by Herod the Great, including the hippodrome. On the lowest level of the seating stands, facing into the actual racing oval, Danny pointed out a series of mosaic panels with pictures. Many are animals, some are abstract. All are colorful and seemed designed to be seen by both the contestants and the viewers on the opposite side.
Danny mused that these might be the ancestors of our modern stadium advertising billboards. Fascinating concept! Seems like the Romans came up with everything.