The Theatre of Epidaurus shows that architecture can work without erecting a building: here the structure is an agreement made with the terrain. The designer (the Younger Polykleitos, according to ancient sources) chooses a steep natural slope and sets the seating tiers into that incline step by step, facing them with stone; the mountain becomes the support of the auditorium. A seating mass for tens of thousands thus rises without a vast load-bearing frame.

The plan is an exercise in pure geometry. The fan-shaped cavea of 55 rows, more than a semicircle, looks onto a circular performance space (the orchestra). The radius, rise, and rake of the rows are set so that the stage is seen without obstruction from every point — the tiering becomes a calculation that guarantees the sightline. Its most discussed feature is its acoustics: sound from the orchestra reaches the topmost row with surprising clarity. This is thought to be because the stone rows filter out low-frequency background noise while reflecting the higher-frequency sounds of speech; acoustics here is not a separate technology but a by-product of the geometry of tiering.

Epidauros establishes the lasting structural model of the open-air collective space — the theatre, then the stadium, the assembly step: a typology that turns terrain into auditorium and geometry into sight and sound. Still in use today, this scheme is the ancient world's purest answer to the problem of the 'building that gathers a community'.