The fourth level presented 80 panels from which corbels were projecting (projecting parts of a wall). These corbels carried almost vertical wooden poles, that surrounded completely the amphitheatre, and allowed to stretch a canvas ( a velum) to protect the spectators from rain or heat. Seamen, who were used to hoist up sails, did the job to fold or unfold the gigantic canvas. A great specialist of this matter, René Chambon, suggests to visit his website, where you will find all details about the remarkable cleverness of Romans. On this copy of a computer-generated image, which I include in my model, you can notice that the circle of milestones don’t appear anymore. Nonetheless René Chambon suggests on his site a more suitable version to his research, i.e. bases for winches to hoist up the poles.