The 'Vauban Fortress' of Foras was initially a strategic fortification established by Philip the Fair, circa 1300. The donjon was rebuilt in 1480-1490 by Jehan II de Brosse.
The creation of the Rochefort Dockyard in 1666 made it a centrepiece of the system for defending the approaches of the Charente, which is prohibited from entering. At the end of the 17th century, François Ferry, the engineer of King Louis XIV, transformed the old feudal residence into the fort. In 1689, Ferry reinforced the walls of the Donjon to set up a battery of 9 canons and a signaling point. In 1693 a lower circular battery was set up to control access to the river Charente. The donjon received a signal station from 1889 to World War II.
Today classified as a Historical Monument, the castle houses a museum of regional history. Free entrance or guided tour of the fortification and the underground parts of the keep at 2 p.m every day (except Monday) between June 1st and the third weekend of September.
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.