Koroinen was the residence of Bishop of Finland between years 1229 and 1300. in 1300 the bishop seat was moved couple of kilometres further down the River Aura, to the present-day Cathedral of Turku. There were at least two wooden churches in Koroinen built in the 14th century. The latest one was probably a stone church. It was later destroyed by the Victual Brothers in 1396.
Archaeologists have found remains of three wall grounds, which were probably part of the church. Today there are a white, wooden memorial cross and some stone foundations still remaining on site. The landscape of the site is anyway unique in Finland. You can see three medieval church from there: The Turku Cathedral, St Mary's Church and St. Catherine's Church.
The Château d'Olhain is probably the most famous castle of the Artois region. It is located in the middle of a lake which reflects its picturesque towers and curtain walls. It was also a major stronghold for the Artois in medieval times and testimony to the power of the Olhain family, first mentioned from the 12th century.
The existence of the castle was known early in the 13th century, but the present construction is largely the work of Jean de Nielles, who married Marie d’Olhain at the end of the 15th century.
The marriage of Alix Nielles to Jean de Berghes, Grand Veneur de France (master of hounds) to the King, meant the castle passed to this family, who kept it for more than 450 years. Once confiscated by Charles Quint, it suffered during the wars that ravaged the Artois. Besieged in 1641 by the French, it was partly demolished by the Spaniards in 1654, and finally blown-up and taken by the Dutch in 1710. Restored in 1830, it was abandoned after 1870, and sold by the last Prince of Berghes in 1900. There is also evidence that one of the castles occupants was related to Charles de Batz-Castelmore d'Artagnan, the person Alexandre Dumas based his Three Musketeers charictor d'Artagnan on.
During the World War I and World War II, the castle was requisitioned first by French troops, then Canadian and British soldiers. The current owner has restored the castle to its former glory.