This opinion led to the thought of finding a good rear defence area where the Estates and the Swedish government could move if the capital was in danger. This in turn naturally strengthened the central defence theory. The core land () would be the place from where the main part of the field army would conduct its operations, supported by a few fortresses. The area between the two large lakes of Vänern and Vättern was the choice for this core land, with approximately the same border as Skaraborg County. Sweden above the province of Dalarna was not even considered to be included in this strategy. The realisation of the middle Sweden central defence system was finally made 5 February 1819 when King Charles XIV John decided that one main fortress, the central fortress, and two smaller "operational fortresses" were to be built. The central fortress became Karlsborg Fortress at Karlsborg, while the two operational fortresses meant to be located at either end of Vättern never were built due to lack of funds.