Finland became part of Sweden during three crusades, undertaken by the Swedes. The first one took place in the middle of the 12th century, when St. Erik and Bishop Henrik marched into south-western Finland. Bishop Henrik later became the Patron Saint of Finland. The second crusade went to Tavastia in 1239 and was led by Birger Jarl. Marshal Torgils Knutson commanded the third crusade, which went to Karelen in the south-east of Finland in 1293.
Eastern Finland lay on the borders between Sweden and Russia. There were often battles and wars on these borders. The Swedes built several castles in the south-east of Finland to protect the frontier against Russian invasions. They erected the castle at Viborg at the end of the 13th century and the castle of Olafsborg in Savonlinna in 1475.
Finland had only a few towns during the Middle Ages. Åbo was the oldest and most important. The region around Åbo was the centre of the country. Viborg was also an influential town which had grown up around the stronghold.
Almost all peasants were freeholders with farms and land of their own.
The most important castles, Åbo, Tavastehus, Viborg and Olafsborg, were handed over to the Swedish aristocracy. Bo Jonssson Grip controlled the whole country in the 1370 ´s. Karl Knutson Bonde, at the middle of the 15th century and Erik Axelsson Tott, at approximately the same time, were other noblemen who were very influential in Finland.
Åbo imported salt and other goods from the German trading association, the Hanseatic League. Fish, hide, fur, butter and seal-oil was shipped to the south.
During the Kalmar Union, at the end of the 14th and the beginning of the 15th centuries, life was peaceful and prosperous in Finland. The Union King, King Erik of Pommern, often stayed in Finland. The country was reformed and new land was cleared. The Black Death did not affect Finland as badly as in the other Nordic countries.