In connection with excavation and restoration work at the castle of Kastelholm, Finland, radiocarbon dating of mortar has been attempted together with thermoluminescence (TL) dating of bricks and roof tiles. From the first dating results, a discrepancy between the traditional picture of the castle and the dates as well as the archaeological findings started to build up. TL dates on bricks and roof tiles are in agreement with corresponding mortar dates. From the dates obtained, two building periods can be distinguished, an older one dating to within the period 1260-1390 and a younger one to within the period 1390-1560. The results obtained are consistent and give support to the chronological frame based partly on archaeological finds and historical data.