Raid on Sigtuna
By XI-XII centuries Karelian and Estonian tribes reach early feudal state and have all preconditions for fulfillment of near and far see forays. According to historic data, Estonian navigation technique was not at all inferior to Swedish. Karelian have been in quite a different situation and could not foray in Baltic Sea. The peculiarity of Karelian situation was that they could expanse from their historical motherland - Karelian Isthmus - only in three out of four directions. Movement to the Northwest lead to serious clashes with kindred Finnish tribes. Even the first mention about Korela (Karelian) is a report about their failure in raid against Emi tribe in 1143-1144. Expansion to the North and East does not meet any serious resistance. And only the fourth - Southwest direction was deadly blocked by The Great Novgorod.
Quite soon Sweden and Novgorod intervened into intestine Finnish-Karelian wars. Novgorod spread its influence on Karelia, formally not a part of the feudal state. Sweden in 1155 organized first Crusade to Finland in 1155 and by the end of XII century settled on the Southeast shores of the Gulf of Bothnia.
After 1170 Swedish troops in their march to the East reached the lands, protected by The Great Novgorod. In 1178 Novgorod organized the raid of its new ally - Karelian - into the Swedish possessions in Finland. During this raid Swedish bishop Rudolf was captured and later killed.
In the summer 1187 Eastern Vikings - Karelian - with the help of other Novgorod allies - Estonian islanders - stoke a lightning raid on one of the few towns of the medieval Sweden - Sigtuna.
Sigtuna, trade and political center of Uppland province, situated on Mälaren Lake, was a bishop residence. Well-equipped wooden walls surrounded the town. From the North the town was protected by mashes and to the South, from the seaside, there were a harbor, protected by chains.
Erik chronicle says about Karelian foray:
They went on a and calm one that in the storm to Mälaren and they stayed quite secretly in the archipelago of Svea usually with the secret army. Once they got such a notion that they burned Sigtuna, burned it along the bottoms and the town did not get help anywhere. The archbishop Jon was killed there and many heathens were happy from it that the Christians were so unlucky. And the whole of Karelia and Russia were happy about it.
Massive copper city gates of Sigtuna were delivered to Novgorod as present or kind of tribute and are still decorating the Western entrance of Veliky Novgorod Saint Sophia Cathedral.
To make Karelian more dependant of the Great Novgorod the Novgorod Prince Yaroslav Vsevolodovitch organized the mass christening of them to the Orthodox Church in 1227-1228. The intestine clashes between Karelians and Finns, that were becoming more and more dependant on Swedes, were intensified by religious contradictions.