Tramontano Castle, begun in the early 16th century by Gian Carlo Tramontano, Count of Matera, is probably the only other structure that is above ground of any great significance outside the sassi. However, the construction remained unfinished after his assassination in the popular riot of 29 December 1514.
It has three large towers, while twelve were probably included in the original design. During some restoration work in the main square of the town, workers came across what were believed to be the main footings of another castle tower. However, on further excavation large Roman cisterns were unearthed. Whole house structures were discovered where one can see how the people of that era lived.
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.