The aisled St. Mary's Church of square ground plan whose lateral walls have been preserved almost to their original height. The size of the church (11x 24 m) indicates a large population of the nearby castrum erecting this edifice in the 5th - 6th centuries.
The altar area is two steps higher and is divided from the church nave by a partly preserved triumphal arch. The altar basis is preserved in situ. The atrium of the basilica holds a small collection of stone monuments, exhibiting, among other, church finds, transennae, stone window grids.
The front part of the basilica and around it was the site of an Early Christian cemetery 300 m long, stretching as far as the southeastern corner of the castrum.
The beginning of the 9th century saw the renovation of numerous churches, among them St. Mary's. It was refurbished with new stone furnishing bearing the recognizable interlace pattern. The large pagan population must have caused missionary activities carried out by the Benedictines from the many monasteries. If not earlier, this is when the Benedictine monastery was founded stretching north and south of the church.
In the 13th century the basilica belonged to the Templars, but when their order was abolished in 1312, life in the monastery faded away.
Near this church, a smaller aisleless church dedicated to St. Peter was built whose floor was decorated by a tricolored mosaic.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.