The amphitheater (or stadium) that Theseus builds for Palamon and Arcite's jousting competition represents Theseus's attempt to contain their lust and violence in a controlled, orderly fashion. The narrator tells us that the stadium is built in tiers "that whan a man was set on o degree, / He lette nat his felawe for to see" (1033-1034). This aspect of the stadium's construction might represent the public nature of the fight that will happen there; it's important that everyone be able to witness the joust and testify to its fairness. (Not sure how to picture what Chaucer describing here? Think of an ancient football stadium, or the Colosseum.)
Theseus orients the stadium with gates at the western and eastern ends. It is a mile in circumference, and to construct it Theseus has hired experts in geometry and arithmetic, as well as painters and sculptors. He has thus gathered all the resources of civilized society – both the mathematicians and the artists – to build his stadium, representing the civilized world's attempt to contain the baser desires of mankind.
Theseus has even built temples to Venus (goddess of love), Mars (god of war), and Diana (goddess of virgins), in the stadium, so that all the forces at work in Palamon and Arcite's joust – love, violence, and the object of desire, the maiden – are represented in its construction.