The 'Old Church' of Diekirch with its two naves in Gothic style, dates back to 1467. In the Merovingian period (6th and 7th centuries), a pre-Romanesque church was built in the place of a Gallo-Roman sanctuary, certain parts of whose walls are still visible today. In the 11th century, this building was replaced by a Romanesque church, whose tour has been kept (12th century). Inside, the church has fine frescoes dating from the 16th and 17th centuries. In 1960, a cemetery, probably from the Merovingian period, was discovered underneath the church.
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.