Aya Trias Basilica was built in the early 6th century and destroyed during the Arab raids of the 7th century. It was then abandoned, and a small church and other buildings were built to the south. These buildings in turn were abandoned and destroyed around the 9th century. All memory of the basilica disappeared, until it was rediscovered by chance in 1957, when it was partially excavated.
What can be seen of the basilica today is an entrance atrium at the western end of the basilica, (the end furthest away from today's entrance gate). This leads to an entrance lobby, or narthex, spanning the width of the basilica. This in turn leads to a three-aisled nave, with a number of columns still standing. here you can see the remains of a chancel. There is a large central apse and two smaller apses to the north and south. To the southeast of the basilica, you can see the remains of a large cross shaped baptismal chamber, the largest known on the island. It is thought that the other structures you can see here are the remains of the Bishop's Palace.
The narthex and nave are extensively covered with mosaics, mostly geometric patterns. In the northern nave, however, there are some exceptions. In particular, a pomegranate tree, alongside a pair of sandals.References:
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.