Saint Martin's Church of Valjala is the oldest church in Estonia. Immediately after the conquest of 1227, a stone chapel was erected by Teutonic knights in Valjala not far from the ancient stronghold. Its walls form the lower part of the present church choir. On the southern side of the chapel, there was a vestry.
Soon after completion, the chapel was decorated with murals, the remaining fragments of which may be seen on the northern wall of the church (six seated apostles in a Romanesque framing). In 1240 construction of the single-nave church was started. The original chapel was transformed into a choir.
Valjala Church was robbed and partially destroyed during the St. George's Night Uprising in 1343 later reconstructed. In the second half of the 14th century, a new polygonal apse was added to the church. The tower, which is curiously located above the vestry on the southern side of the church, was probably not completed until the 17th century. In the walls of the tower, fragments of archaic trapezoidtombstones can be seen. Archaic tombstones of this type have previously only been found in western Estonia. They might originate from the pre-Conquest period.
The Kuressaare master, Nommen Lorenzen, made the altarpiece in 1820. Besides a medieval baptismal font, other noteworthy antiques inside the church are two Baroque epitaphs (of Andreas Fregius, 1664 and Gaspar Berg, 1667).
In 1888 Gustav Normann built the church's organ. The lightning struck and burned part of the roof in 1922. Dolores Hoffman, who began work on them in the 1970s, made the stained glass windows. They mark the beginning of a new tradition in Estonian stained glass art.
The most impressive elements in the interior design of the church are the high domed vaults with Westphalian ribs and bosses. A ridge in the wall and the remains of girders are evidence of a defense gallery that once ran beneath the windows of the nave. The doorway in the intrados of the chancel arch led to the loft, which served as a refuge.
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.