The Roman Odeon, the most significant ancient monument in Patras, is in the upper town and was built around 160 AD, in the reign of either Antoninus Pius or Marcus Aurelius. It was severely destroyed by successive invasions, wars, and earthquakes. It was almost buried under the remains of other buildings and ground. It was in 1889 when the Odeon was found by accident while some workers were digging up the ground for the construction of the port. It has been restored and partially reconstructed, and is used as an open-air theatre for performances and concerts in the summer.
The Church of St Donatus name refers to Donatus of Zadar, who began construction on this church in the 9th century and ended it on the northeastern part of the Roman forum. It is the largest Pre-Romanesque building in Croatia.
The beginning of the building of the church was placed to the second half of the 8th century, and it is supposed to have been completed in the 9th century. The Zadar bishop and diplomat Donat (8th and 9th centuries) is credited with the building of the church. He led the representations of the Dalmatian cities to Constantinople and Charles the Great, which is why this church bears slight resemblance to Charlemagne's court chapels, especially the one in Aachen, and also to the Basilica of San Vitale in Ravenna. It belongs to the Pre-Romanesque architectural period.
The circular church, formerly domed, is 27 m high and is characterised by simplicity and technical primitivism.