Helme church parish was first mentioned in 1329 during a Lithuanian raid. Most of the neighbouring land was fiefed in the 15th–16th centuries. Livonian Order castle in Helme (Ordensburg Helmet) was probably built in the first half of the 14th century. The site on a steep hill is belived to have been used as a stronghold earlier by Sackalians in the Ancient Estonia. By its ground plan the order castle was 120×60 m oval shaped structure, surrounded by two moats. There was a borough beside the castle in the Middle ages, it was mentioned until the 17th century. The castle was destroyed by the Swedes during the Russo-Swedish War in 1658. Nowadays only parts of the walls with high window holes have survived.
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.