What is the world's oldest indoor theatre?

The Teatro Olimpico is a theatre in Vicenza, northern Italy, constructed in 1580-1585. The theatre was the final design by the Italian Renaissance architect Andrea Palladio and was not completed until after his death.

In 1555, a group of artists and noblemen founded the "Accademia Olimpica". In 1579, Palladio, as one of its members, proposed the construction of a permanent theatre for dramatic presentations, as the wooden structures he had designed for the Palazzo della Ragione hall had been dismantled. Palladio's proposal was accepted and, in February 1580, construction work began in the courtyard of the 13th-century territorial castle. Andrea Palladio died a few months later and the work was continued by his son, Silla, but direction of the project was soon after entrusted to Vincenzo Scamozzi. The Olimpico is a revival of the structure of Roman theatres, but the cavea or auditorium, squashed for reasons of space, is semi-elliptical instead of semicircular. The Frons Scenae, or stage, is in the shape of a triumphal arch. The central arch seems to be its "Porta Regia", but at the same time presents the tripartite form of the grand façades of Palladian palaces.