South - Eastern Finland fortification system South - Eastern Finland fortification system is an extensive defensive system formed by three concentric fortress chains in South - East Finland built by Russia in 1790s . The purpose of the fortification system was to protect the capital of the Russian Empire , Saint Petersburg , from a possible Swedish attack . Russo - Swedish War of 1788 – 1790 and the defence of Saint Petersburg . Before the South - Eastern Finland fortification system was built , the defence of the Russian controlled South - East Finland or Old Finland in 18th century relied mainly on the fortresses captured from Sweden . Russia had acquired the medieval castles in Vyborg ( Viipuri ) and Korela ( Käkisalmi ) in the Treaty of Nystad 1721 . In the 1743 Treaty of Åbo Sweden ceded the 17th - century fortress in Lappeenranta and the Hamina fortress built in 1720s as well as the medieval castle of Olavinlinna in Savonlinna to Russia . The only new fortress built by Russia in Old Finland before 1790s was the construction of the first phase of the Taavetti fortress in Luumäki between 1773 – 1781 . The Russo - Swedish War of 1788 – 1790 started by the new King Gustav III of Sweden ended in the Treaty of Värälä with neither side gaining territory . Nevertheless , the victories the Swedish army achieved in the war had shown that the defences of the North - Western border of Russia were inadequate and the capital of Saint Petersburg – located relatively close to the border – was thus in potential danger . After the war Russia began to strengthen the defences in Old Finland . The empress of Russia , Catherine II , gave the responsibility of strengthening the defences by creating a strong fortification system to General Alexander Suvorov in 1791 . Three concentric fortress chains . General Suvorov rapidly started the construction works . The objective was close all roads leading to Saint Petersburg through Old Finland with fortresses . New fortresses were built at cross roads or in terrain that was difficult to bypass and old fortresses were renovated according to the latest fortification technology . The end result was a strong fortification system of three concentric fortress chains . The inner fortress chain closest to the capital was formed from Shlisselburg fortress ( Pähkinälinna ) , Kronstadt fortress and the castles of Vyborg and Korela . The second chain was formed from the fortresses of Hamina , Taavetti and Lappeenranta . New border fortresses built in Kymenlaakso formed the outermost chain along with the medieval castle of Olavinlinna . The inland fortresses of Kärnäkoski , Järvitaipale and Utti along with the fort of Liikkala were quite small , but the double fortress of Kyminlinna and Ruotsinsalmi sea fortress on the coast was quite formidable . Only the fortresses in the outermost chain were new , but existing older fortresses were also enlarged and strengthened . The earth walls of Hamina and Lappeenranta were reinforced with stone walls , Taavetti fortress was enlarged , the cannon towers of Olavinlinna were raised and bastion s were added to the medieval castle . The bastions of Vyborg and Korela castles were also reinforced . In addition to the fortifications , the ability of the Russian Navy to sail in Saimaa Lake was improved by building four new open canals called Suvorov Military Canals on the waterway between Lappeenranta and Savonlinna . The canals were needed , because Sweden could still control the water traffic in Saimaa at the narrows in Puumala . The canals of Kutvele , Käyhkää , Kukonharju and Telataipale located in Taipalsaari , Ruokolahti , Puumala and Sulkava respectively were built alongside the fortresses . Gunboats of the Russian Saimaa Fleet were based at Lappeenranta and Kärnäkoski fortresses and Olavinlinna castle . The fleet consisted of less than a hundred different sized vessels and one thousand men . Harsh construction work . The manpower for the construction works was provided by the regular army of the Vyborg Governorate ( 20 000 men ) and a temporary Finland division ( Finljandskij division ) of 20 000 – 30 000 men . Prisoners were also used for labor . The smaller border fortresses were finished quickly , some already in the early 1790s . Larger construction works , such as canals , took longer . The conditions in the construction sites were harsh . Severe climate , poor accommodation , insufficient nutrition and sickness took a toll on the men . Up to 10 000 men died or were too ill to work . The gruelling pace of the work maintained by General Suvorov is also said have caused the death of many workers . The local Finnish population did not have to partake on the actual construction work , but had to provide shelter , food and transport for the large army . Fortresses of South - East Finland as protection of Russia . The enormous amount work done to fortify Old Finland was wasted , as the border between Sweden and Russia moved westward as a result of the Finnish War between 1808 – 1809 . The fortification system was not tested in this war , as the Swedish troops retreated from border well before Russians attacked . After the war the inland fortresses were abandoned , and only the coastal fortresses retained their military value . Ruotsinsalmi sea fortress was attacked by British - French fleet during the Crimean War in 1855 . The Russian troops abandoned the fortress before the attack and the enemy demolished the empty sea fortress . Present - day condition of the fortresses . Most of the fortresses are now popular tourist attractions . Fortresses inside the borders of modern Finland are maintained by the Finnish National Board of Antiquities , which has been restoring them especially during 1990s . The last part of the fortification system , the military canals of Suvorov , are currently under restoration work .