Saint William's Church is a gothic church is known for its sumptuous interior combining the Gothic and Baroque styles. Since the end of the 19th century, the excellent acoustics of the church has allowed it to serve as a venue for concerts of classical music, in particular for the Passions of Johann Sebastian Bach.
Returning unharmed from the Crusades, the knight Henri de Müllenheim undertook the construction of a monastery for the Hermits of Saint William, an order of mendicant monks, in this marshy neighbourhood situated extra muros, that is, beyond the city walls. The elongated building, consecrated in 1301 and realised in 1307, is the only remnant of this group. Entirely brick and unvaulted, the church corresponds well to the ideal of the order, namely by its single nave and the simplicity of its exterior form. Sheltered by a pitched roof, its nave is topped and prolonged by a deep polygonal choir illuminated by high windows, which betrays its original function as the monks' meeting room. In 1331, by reason of its proximity to the port and wharfs, the church was chosen as parish by the newly established corporation of shipbuilders.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.