Svartholma fortress has no great military achievements to boast of. The first occasion on which Svartholma fell under the threat of attack by the Russians was during the coup to install King Gustav III in 1772. In the end, the international political tensions were relieved without a military confrontation.

During Gustav III:s war against Russia in 1788–1790, Svartholma fortress was not involved even though the front was only 10 kilometres or so to the east. Russian naval divisions avoided the Degerbynlahti bay in order to remain beyond the reach of the guns in Svartholma, sailing along the coastline all the way to Hanko.

After the war, the Russians built the Ruotsinsalmi sea fortress off the mouth of the river Kymijoki, as well as the Kyminkartano land fortress. This meant that there was a significant sea fortress on both sides of the border. However, when the Finnish War broke out in 1808 Svartholma was poorly equipped and its shoreline defences were incomplete. Its armaments were weak, as only a third of the required number of guns were in place and most of its small arms were not in working order. Food supplies were deficient and the fortress' well was poorly managed. The sick bay was very small and also poorly run. As more reinforcements were transferred to the fortress, the total number of men climbed to around 700. Women and children who resided in the fortress were sent away.

The Russians crossed the border only a week after the outbreak of the war and conquered Loviisa without a fight. They began to prepare the siege of Svartholma. Fearing large losses, the Russians abandoned a direct attack over the ice. Approximately 3,000 Russians besieged the fortress and attempted to starve the garrison into submission. The besiegers focussed on negotiations and psychological warfare rather than military action. For instance, they made tempting offers to the officers defending the fortress.

The Russians fired their guns only occasionally. The resulting damage was minor, including only a few broken windows and a fallen chimney. This proved decisive, however, as the buildings could no longer be kept warm and the winter was freezing. The defenders' situation was made worse by the fact that the guns in the fortress did not fit the embrasures properly, making it impossible to counter the Russians' gunfire. The willingness to defend the castle quickly evaporated as provisions and drinking water became scarce, medication ran out and an increasing number of men fell ill. As the officers' will to fight declined, Sergeant Major Edlin initiated a mutiny to usurp the officers who were planning to surrender. The mutiny was suppressed and Edlin was sentenced to death, but was pardoned later.

By Regular Water Bus Service