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:
First record of Kastelholma (or Kastelholm) castle is from the year 1388 in the contract of Queen Margaret I of Denmark, where a large portion of the inheritance of Bo Jonsson Grip was given to the queen. The heyday of the castle was in the 15th and 16th centuries when it was administrated by Danish and Swedish kings and stewards of the realms. Kastelhoma was expanded and enhanced several times.
In the end of 16th century castle was owned by the previous queen Catherine Jagellon (Stenbock), an enemy of the King of Sweden Eric XIV. King Eric conquered Kastelholma in 1599 and all defending officers were taken to Turku and executed. The castle was damaged under the siege and it took 30 years to renovate it.
In 1634 Åland was joined with the County of Åbo and Björneborg and Kastelholma lost its administrative status.