Kärnäkoski Fortress is part of the outer chain of fortresses in the fortification system built in Southeastern Finland to protect St. Petersburg. The chain comprised the fortresses of Kyminlinna, Liikkala, Utti, Järvitaipale, Kärnäkoski, Partakoski, and Savonlinna. The fortress was built between 1791 and 1793. The plant species found in the inner yard of the fortress have been listed and include 130 different vascular plants, seven of which are originally from Russia.
Kärnäkoski Fortress includes a hillside fort outside the main fortress. The fort can be reached along a narrow path. In the past, a guard was able to see any passers-by from the fort, which offered an excellent view all the way to Kärnäsalmi Bay towards Lake Kuolimo.