Inspired by Greek theatres, which had a similar structure, ruins of Ancient Roman theatres can be found throughout the territories once ruled by the Empire. Typical Roman theatres were constructed of concrete, a material no longer used in Renaissance Italy because the recipe had been lost. Their semicircular design created superior acoustics, allowing spectators to hear the plays, speeches, or choral events performed on stage.
Notice the lack of marble? Renaissance Italians practiced a rudimentary form of recycling when dealing with Roman structures: any marble that could be found was ripped of the buildings to be reused for free.