After the Russo-Swedish War (1741–1743) Sweden ceded the Lappeenranta fortress and Olavinlinna castle in Savonlinna to the Russian Empire in the Treaty of Åbo. The treaty divided Saimaa lake in two, with Sweden controlling the strait of Puumala and requiring toll payments from water traffic between Savonlinna and Lappeenranta and denying military traffic altogether. After the Russo-Swedish War (1788–1790) Russia began to improve the defences in Finland by building the South-Eastern Finland fortification system. The war had also showed the importance of Saimaa water ways and as part of the fortification system it was decided to construct a series of canals to bypass Puumala.