After the Sweden's defeat in Russo-Swedish War 1741-1743 (also known as the Hats' Russian War) eastern border of Finland was moved to west. Important fortresses of Hamina, Lappeenranta and Savonlinna were left to Russian side of border.
The city of Loviisa was established in 1745 to handle a international commerce in Finland. Planning of the new fortification system started concurrently, because Loviisa was located alongside the strategic road from Vyborg to Turku. The parliament accepted the plan of twin fortresses in 1747: one in Loviisa city to protect the road and another to Svartholma island to defence Loviisa from the sea. Building of the Loviisa fortress started in 1748, but the plans were never completed. The ground was too muddy for heavy stone walls and the Crown had continuous lack of money. Building was interrupted in 1757 and the king Gustav III ordered to stop the construction permanently in 1775. Loviisa fortress didn't see any battle. In the Finnish War 1808-1809 it surrended to Russians without fighting, because people of Loviisa were afraid of the destruction of the city.
Nowadays there remains two completed bastions, Rosen and Ungern. Renovation started in the 1960's and Ungern is mostly renovated. Rosen is still uncompleted and mostly ruined.
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.