The Apollon Theatre on Georgiou Square is the oldest municipal theatre in Greece, opened in 1872 in an Italian neoclassical design. The exterior is handsome in a restrained way that suits the square it faces, but the interior is the reason to seek it out: the horseshoe-shaped auditorium with three tiers of boxes and a painted ceiling is one of the finest theatre interiors in Greece, modelled loosely on La Scala in Milan. It seats around 800 and has been functioning continuously as a performance space for well over a century.
The theatre hosts opera, drama, and concerts throughout the season, with the Patras Carnival using it as one of its main venues. If you're in Patras and there's a performance on, it's worth going — the combination of a working 19th-century theatre and a Greek audience that takes its culture seriously makes for an excellent evening. Even outside performance times, the exterior is worth a look as part of a walk around Georgiou Square, and guided tours of the interior are occasionally available. Check current programming at the city's cultural office or at the theatre box office.