The fortification system

Work on the fortifications of Viapori began in 1748, applying the bastion fortification system developed in Europe in the late 17th and early 18th century. Viapori was laid out as an irregularly traced fortification on several islands. This was because the fortifications had to be built according to the local terrain, and in many cases excavated directly into the rock. 

In the final plan, the fortress is divided into a main fortress and surrounding outer fortresses. The main fortress consists of Iso Mustasaari and Susisaari islands, while the outer fortresses are on Kustaanmiekka, Pikku-Mustasaari, Länsi-Mustasaari, Särkkä and Vallisaari islands.

By the time Viapori was taken over by the Russians, the bastion system had become outdated. After the outbreak of the Crimean War (1853–1856), the Russians began to build new gun batteries on Vallisaari, Kuninkaansaari and Santahamina islands. Work was also begun on a new chain of defences around the city of Helsinki. Viapori was considerably expanded during the Crimean War, but its military capabilities were shown to be inadequate during the bombardment by the Anglo-French fleet.

Towards the end of the 19th century, the fortifications were strengthened by covering them with sand. Other active repairs were undertaken before the turn of the century. In the 20th century, however, repairs subsided, and the fortress became hopelessly outdated in terms of armaments compared with the constant rearming of the world’s superpowers.

During the First World War, the defence perimeter of Viapori was pushed outwards by a few kilometres; a new line of defences was built on Harmaja, Kuivasaari and Isosaari islands. Fortifications were also built on the mainland. By the end of the war, there were three concentric lines of defence around Helsinki. The Russian Revolution halted work on the Viapori fortifications, and in spring 1918 the fortress was taken over by the newly founded Finnish government.