The sanctuary on Mount Sant'Angelo is southern Lazio's balcony: the great arcades of Sulla's era — the "temple" seen floodlit from all Terracina — supported a sacred terrace whose gaze runs from the Circeo headland to the Pontine islands. The complex, tied to the cult of the boy Jupiter (Anxur), enchanted Grand Tour travellers already.
The cryptoporticus and the small temple's remains are visited; the panorama does the rest.
Our tip: climb an hour before sunset: the sun drops behind the Circeo exactly opposite the terrace, and the arcades ignite. In summer, check the evening openings: night up here is pure stagecraft.