Kirkkosaari ("The Church Island", also known as Köyliönsaari) is one of the oldest places of residence in Satakunta area. Two Iron Age cemeteries are located at the north side of Kirkkosaari.
According the legend Lalli, the pagan chief of Kirkkosaari manor, killed bishop Henry on the ice of lake Köyliönjärvi on January 20, 1156. Although Henry has never been officially canonized, he has been referred to as a saint since as early as 1296 according to a papal document of the time. After this the manor and island was moved as the property of Turku bishop. Since 1746 the manor has been owned by Cedercreutz family and is still in private use. Present buildings are from the 18th and 19th centuries.
The church of Köyliö has been in Kirkkosaari since the Middle Ages. The present wooden church was built in 1752 to the old cemetery site. There are also some remains of the medieval chapel in Kirkkosaari manor garden.
Sweetheart Abbey was a Cistercian monastery, founded in 1275 by Dervorguilla of Galloway in memory of her husband John de Balliol. His embalmed heart, in a casket of ivory and silver, was buried alongside her when she died; the monks at the Abbey then renamed the Abbey in tribute to her. Their son, also John, became king of Scotland but his reign was tragic and short. The depredations suffered by the Abbey in subsequent periods, have caused both the graves to be lost. The abbey, built in deep-red, local sandstone, was founded as a daughter house to Dundrennan Abbey; this Novum Monasterium (New Monastery), became known as the New Abbey.
The immediate abbey precincts extended to 120,000 m2 and sections of the surrounding wall can still be seen today. The Cistercian order, also known as the White Monks because of the white habit, over which they wore a black scapular or apron, built many great abbeys after their establishment around 1100. Like many of their abbeys, the New Abbey's interests lay not only in prayer and contemplation but in the farming and commercial activity of the area, making it the centre of local life. The abbey ruins dominate the skyline today and one can only imagine how it and the monks would have dominated early medieval life as farmers, agriculturalists, horse and cattle breeders. Surrounded by rich and fertile grazing and arable land, they became increasingly expert and systematic in their farming and breeding methods. Like all Cistercian abbeys, they made their mark, not only on the religious life of the district but on the ways of local farmers and influenced agriculture in the surrounding areas.
The village which stands next to the ruins today, is now known as New Abbey. At the other end of the main street is Monksmill, a corn mill. Although the present buildings date from the late eighteenth century, there was an earlier mill built by and for the monks of the abbey which serviced the surrounding farms.