The Kostelec nad Černými lesy Castle is the main landmark of the town. The castle was first documented in 1348. Until 1549, when it was destroyed by fire, the castle served as a hunting lodge of Emperor Ferdinand I. After 1550, it was rebuilt in the Renaissance style. After 1750, baroque modifications were made. Today the castle is owned by Czech University of Life Sciences Prague and is used for social and business purposes. The neighbouring Church of Saint Adalbert is connected with the castle by a covered corridor.
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.