In 1746 Brynolf Brunou was granted to establish an ironworks to the Juankoski rapids. Juankoski was in the periphery of Sweden borderlands with no roads or cities. Water routes were the only way to transport goods. Juankoski became the only ironworks in Finland which used only the bog iron to manufacture different kind of tools.
The heyday of Juankoski was between 1851 and 1900 when it was owned by Anastasia Ponomareva, a Russian aristocrat. In her time the manufacturing process became more efficient and better transport routes were built to ship products. The village around grown to 1,000 inhabitants. The iron manufacturing ended in 1915 when Juankoski was acquired by Kemi Oy and moved to carton mill.
The ironworks area is a well-preserved sample of the industrialism history in Finland. The oldest building is the empire-style mansion built in 1826. The blast furnace is today a museum exhibiting the life of the ironworks society. There are also a bakery, cafe and handicraft shops. Guided walking tours are available in summer season.
Ängsö Castle was first named as "Engsev" in a royal charter by king Canute I of Sweden (r. 1167-1196), in which he stated that he had inherited the property after his father Eric IX of Sweden. Until 1272, it was owned by the Riseberga Abbey, and then taken over by Gregers Birgersson.
From 1475 until 1710, it was owned by the Sparre family. The current castle was built as a fortress by riksråd Bengt Fadersson Sparre in the 1480s. In 1522, Ängsö Castle was taken after a siege by king Gustav Vasa, since its owner, Fadersson's son Knut Bengtsson, sided with Christian II of Denmark. However, in 1538 it was given by the king to Bengtsson's daughter Hillevi Knutsdotter, who was married to Arvid Trolle.
In 1710, the castle was taken over by Carl Piper and Christina Piper. Ängsö Castle was owned by the Piper family from 1710 until 1971, and is now owned by the Westmanna foundation. The castle building itself was made into a museum in 1959 and was made a listed building in 1965. It is currently opened to visitors during the summers.
The castle is a cubical building in four stores made by stone and bricks. The lower parts is preserved from the middle ages. It was redecorated and expanded in the 1630s. The 4th storey as well as the roof is from the expansion of Carl Hårleman from 1740-41. It gained its current appearance in the 1740s.