Kärnäkoski fortress was part of the South-Eastern Finland fortification system built by Russians in the 1790's. Purpose of the fortress was to protect the strategic road to St. Petersburg and Russians fleet in Saimaa against Swedish enemies from the west. Building was started in 1790 by French engineer officers and it's a tradional French bastion system. Approximately 1400 Russian soldiers and local peasants were forced to construction work and many of them died in hard work and illnesses.
Kärnäkoski protected the border only 15 years and after Finnish War in 1808-1809 fortress lost its defensive value. Kärnäkoski was disbanded in 1835 by the tsar Nicholas I. After that fortress was disarmed, buildings and remaining equipment were auctioned and the walls and fortifications were left untended. Kärnäkoski never took part to a military action, but in Finland Civil War (1918) several battles were fought nearby.
Today Kärnäkoski fortress is a tourist attraction, although there are no guided tours or other tourist or travel services in the fortress, simply guidance signs. Finnish National Board of Antiquities and Finnish Ministry of the Environment have listed the fortress area as nationally significant cultural historic landmark. NBA has restored Kärnäkoski fortress to its former shape together with other fortresses in south-eastern Finland. Walls were repaired and the fortress area was restored and cleared. Other historical buildings nearby are an old barge harbor, mill and saw built in 1830s and a double-arch stone bridge from 1886. The mill has not been used since 1950s, but was restored by a local village organization in 2002.
The small Grisslehamn village is today a well-preserved sample of 19th century architecture and popular attraction for daily trips. The name Grisslehamn was first mentioned in a document from 1376 about the mail route between Sweden and Finland. This Grisslehamn was located some 20 km south of today's location. In the mid-18th century, most of the old village was destroyed in a fire, and it was decided to move Grisslehamn to its current location to make the mail route shorter. Conveying mail by row boat from Sweden to Åland, from whence it was transported to the Finnish mainland, was, together with fishing, one of the most important sources of income for the inhabitants of Grisslehamn and other parts of Roslagen for a long time, until steam ships took over the mail routes in the early 20th century.
During the Finnish War in 1809 a small unit of Russian cossacks attacked to Grisslehamn over the frozen Gulf of Bothnia. In the battle 80 Swedish soldiers surrended and several were killed.
Today there are several buildings dating from the 19th century, like the toll station and old barracks. The post office was built in 1755 and the chapel in 1909.