Finland belonged to Sweden throughout the Middle Ages. Finland was the eastern-most part of the realm and was conquered by Sweden during three crusades in the 12th and 13th centuries.
The centre of Finland was at Åbo, a town with a castle and a Bishop. Viborg, on the Russian border was also an important town. Almost all peasants in Finland were free-holders with farms of their own.
The Kalmar Union brought Finland peace and prosperity. In the 15th century, new farm-land was cleared and the taxes were more equally distributed. Members of the Swedish aristocracy dominated the castles.