Vyborg Castle Vyborg Castle ( , , ) is a Swedish built medieval fortress around which the town of Viborg ( today in Russia ) evolved . Currently it serves as a museum . General . Viborg Castle was one of the three major castles of Finland . It was built as the easternmost outpost of the medieval Kingdom of Sweden : it is located on the Karelian isthmus , on a little islet in the innermost corner of the Gulf of Finland , in a tight strait which connects Suomenvedenpohja to Bay of Viipuri . It was originally constructed in the 1290s . The town was originally located inside the outer fortifications of the castle , at the fortress island , but it had to be moved to its present location out of the island because of lack of space . Medieval history . The construction of the fortress started in 1293 by orders of Torkel Knutsson , the Lord High Constable of Sweden who made in 1290s a so - called crusade to Karelia , the so - called Third Finnish Crusade , actually aimed against Russians , i . e . Novgorod . He chose the location of the new fortress to keep the Bay of Vyborg , which was a trading site used by locals already for a long time . From the bay , a river way goes inland , ultimately connecting the place to several districts , lakes , and indirectly also to rivers going to Ladoga . The three high - medieval Finnish " castle fiefs " were ruled from the castles of Turku , Hämeenlinna and Viipuri , respectively until the 1360s . In 1366 , Albert III of Mecklenburg was elected king , and according to the German model he began to divide up the castle fiefs , which were immense in Finland , into smaller provinces . In some cases , these new , smaller provinces were given separate bailiffs ' residences which have since disappeared ; this was the case with the Linnavuori castle mound in Porvoo and the castle of Korsholm in Ostrobothnia , near the today city of Vaasa which was founded later . Only earthworks remain at these sites , but in two places , stone strongholds were built which still exist , although in ruins . One is Raseborg Castle in western Uusimaa ( west of Helsinki ) , the other Kastelholm castle in the Åland ( Ahvenanmaa ) islands The castle became the stronghold of the Swedish realm in Karelian regions . Throughout the centuries , it was the first defence of the kingdom against Russians . Its military and strategic status was in the late Middle Ages only second to the fortified capital Stockholm . The castle and the large surrounding fief became a virtually autonomous principality . Its governors were usually fiefed with the incomes of the county . The fief of Viborg became known as a margraviate . Its governors were generally from the most powerful families of the kingdom . They enjoyed large administrative powers and a good distance from the capital . Those realities made them practically independent rulers . Usually , the castle of Olavinlinna ( built in 1470s ) was subjugated to Viipuri . Prominent figures who held Viipuri as their fief , were Bo Jonsson Grip , Christer Nilsson Vasa ( 1417 – 42 ) , Karl Knutsson Bonde ( 1442 – 48 , the future king ) , Eric Axelsson Tott ( 1457 – 81 ) , Knut Posse ( 1495 – 97 ) , Sten Sture the Elder ( 1497 – 99 , between his regencies ) , Eric Bielke and count John of Hoya . Particularly in 1440s and in late 15th century , the fortresses were further enlarged . The first mention of firearms in Finland relates to Viborg castle , in 1429 . During the Middle Ages the castle was repeatedly besieged by the Russians , most famously in 1495 , during the Russo - Swedish War ( 1495 – 1497 ) . Governor Knut Posse was in office 1495 - 1497 . The situation of the defenders looked hopeless , but they were saved by the Viborg blast on 30 November 1495 , a mysterious explosion which scared off the Russians because they reportedly saw a St . Andrew ' s cross in the sky . Modern history . In the 16th century , much was renovated and additions made . In the 17th century , the castle was allowed to decay , as Russian danger was decreased and the border was much more eastwards . Viipuri was taken by the Russians in 1710 , but passed back to Finnish hands in 1812 when all of Old Finland was attached to the autonomous Grand Duchy of Finland . The castle owes its present appearance to extensive restorations undertaken in the 1890s . The military of the Russian Empire used the castle until 1918 for housing administration . Viipuri belonged to the independent Republic of Finland between 1917 – 1940 and again 1941 - 1944 . As a result of border changes in World War II it was annexed by The Soviet Union in 1944 . Architecture . The main castle , located in the eastern part of the islet on its highest hill , has an irregular four - cornered layout , with the immense tower of St . Olav ( Pyhän Olavin torni in Finnish ) as its biggest section . It is 3 - 4 stories tall , varying in places . Outer defensive works surround the main castle , following the islet ' s coastlines . As of January 2013 , images of the castle and grounds could be viewed using Google Street View .