Kärnäkoski fortress, located in Kärnäkoski, Savitaipale, is a bastion fortress built by the Russians between 1791 and 1793 as part of the defense system for St. Petersburg. However, its military significance was short-lived as the border between Sweden and Russia shifted far away from Kärnäkoski in 1809. The fortress, designed in the French bastion style, featured irregular shapes and was equipped with multiple bastions, ravelins, and gun emplacements. Despite its brief military use, the site is now a designated cultural heritage area with limited tourist facilities, maintained by the Finnish Heritage Agency.

In the summer, sheep graze freely in the fortress area and take care of the grassy fields. There is a boat dock by the fortress shore.

Kärnäkoski is one of the rapids along which the Kuolimo River flows into Lake Saimaa. Here you can also find the 19th-century mill and sawmill. Another side of the road, from a small path away, is also lake with an old lotja harbor, where you can catch glimpses of the submerged wooden structures of sunken boats in the clear water, and at the fireplace you can enjoy your packed lunch while taking in the lake view.