The theatre at Delphi sits above the Temple of Apollo on the sanctuary hillside, built from local limestone in the 4th century BC and restored by the Romans in the 2nd century AD. It held around 5,000 spectators and was used for the musical and dramatic competitions of the Pythian Games — the second most prestigious festival in the ancient world after the Olympics. The acoustics are good, and the occasional modern performance still uses the space.
What makes the theatre exceptional is not the building itself — it is preserved well but is not extraordinary by Greek theatre standards — but the view from the upper tiers. You sit looking down over the entire sanctuary: the Temple of Apollo in the middle ground, the valley of Phocis dropping away beyond it, and the olive groves extending for miles towards the Gulf of Corinth. It is one of the great panoramas of Greece, and it is not accidental: the Greeks chose this hillside because the landscape itself was part of the experience.