In the early 18th century Russia founded St.Petersburg and began to build a Baltic fleet, threatening Sweden's interests in Finland. In 1721, Sweden lost the vital fortress of Vyborg to the Russians, leaving the eastern frontier defenceless and the Russian fleet a free reign in the Gulf of Finland. This alarmed not only the Swedes but also their French allies, who put pressure on them to fortify the new frontier, even to the extent of offering to finance the new fortifications. In 1747 the Swedish parliament decided to construct a large fortress in the archipelago off Helsinki.

Map of the fortifications of Suomenlinna.

Construction on the fortress, known as Sveaborg in Swedish, began in 1748 under the Swedish military engineer Augustin Ehrensvärd. He became the governor of the fortress, and he continually expanded it until his death in 1772.

The fortress of Sveaborg was built on 6 small islands within sight of Helsinki. The fortress was also to be a naval base, and to this end extensive shipyards and a huge dry dock on the island of Susisaari were constructed. A fleet of special ships designed to operate in the archipelago was built there.

Relief map, view of the distinctive star-shaped fortifications on Kustanmiekka island.

Iso Mustasaari

The main parts of the fortress were situated on the two islands Iso Mustasaari and Susisaari between which were the docks and shipyards.

The outer fortifications on Iso Mustasaari, with the church tower in the background.

Iso Mustasaari was initially fortified with a bastioned trace running round the edge of the island, but no defences on the west, the side facing across the small channel between the two islands, where the shipyards were located. The church, which doubled as a lighthouse, is on this island.

Later in the 18th century, the fortifications of Iso Mustasaari were reinforced with some inner fortifications. A crownwork was built to protect the shipyards, althought it was overlooked by the high ground in the centre of the island.

The crownwork was started shortly after Ehrensvärd's death and it was named after him. Instead of the French-style ramparts of earth revetted by stone, it took the form of a two storey building, with casemates for guns in the flanks of the bastions and loopholes for musket-fire in the curtain walls.

The Ehrensvärd crownwork.

A hornwork was constructed to protect the high ground on the south-west side of the island from which the fortifications on the adjacent island of Susisaari could be easily bombarded. It was partially carved out of the rock, and was defended in the rear by a loopholed redan.

Here, the barrack buildings form the bastioned trace.

At some point, probably after the initial construction period, extra barrack buildings were constructed on Iso Mustasaari. Some of these buildings are unusual in that they form part of the defences, following the bastioned trace. See left - the original wall is the grey stone.


Susisaari was the central part of the fortress and was completely fortified, even the side facing the dry dock and the shipyards. The fortifications overlooking the docks are tall and have lots of gun embrasures that would be able to cover any ship in the dock area.

Left: The fortifications of Susisaari. Right: The enormous dry dock.

Those fortifications would not have faced an infantry assault, only inaccurate naval gunnery fired from swaying ships, unlike the fortifications on the southern side, where there was open ground.

The unusual demi-lune, Hyvä Omatunto.

This section of the defences contains a horn with an unusual demi-lune, which has two demi-bastions and another demi-lune on top of it. The theory was that an enemy attacking the large demi-lune would find himself faced with a minature bastioned system inside it.

At the time these works were being constructed, Ehrensvärd was receiving criticism, so he named the demi-lune Hyvä Omatunto (good conscience) and the demi-bastions Hyve (virtue) and Kunnia (honour).

Another of Ehrensvärd's inventions is the tower behind the gate protected by Hyvä Omatunto. The idea was that if the attackers broke through the gate, they would be faced by an angled tower immediately in front of them, forcing them to go left or right. Whichever way they went they would be raked by fire from the tower and the outer walls.

Inside the gate. Left is the tower and ahead are some inward facing loopholes, the gate is to the right.


The southern front of the main fort on Kustaanmiekka.

The island of Kustaanmiekka used to be seperated from Susisaari by a shallow stretch of water, but the two islands have since become one. The high ground on the island could have been used to bombard the central fortress on Susisaari, so it had to be protected.

Ehrensvärd built a strong bastioned fort, which was smaller than the other fortifications of Suomenlinna; it had no important buildings or shipyards to protect, its sole purpose was to defend Kustaanmiekka. An interesting feature of this fort is its double southern front. There is a ditch across the middle of the fort and an inner line of defence.

On the southern end of the island there was a star shaped line of casemates following a tenaille trace (see right). These casemates overlook the Kustaanmiekka straights and the guns in them could pulverise any ship sailing through them.

The star-shaped fortifications on the south end of Kustaanmiekka.

On the other side of the straights is the island of Vallisaari, which was too large to fortify entirely, but a small work was built there to prevent an enemy gaining a logement from which to attack Kustaanmiekka (see the map at the top of the page)

The King's Gate.

The King's Gate, an decorated ornamental gate, was set in the walls on Kustaanmiekka on the spot where King Frederik landed on a visit the fortress in 1752. Ehrensvärd inscribed the words: "Coming generations: stand here upon your own ground, and never rely on outside help."

Pikku Mustasaari & Länsi Mustasaari

The fortifications on the two islands to the north, Pikku Mustasaari & Länsi Mustasaari, completed the defensive ring around the dockyards. To sail into the dockyards from the north, ships would have to come under the guns of either or both of these islands.

The fortifications on Pikku Mustasaari.

Both islands had barrack buildings on them and even today Pikku Mustasaari is home to a naval academy.

Pikku Mustasaari as seen from Länsi Mustasaari.

Pikku Mustasaari had a roughly square, bastioned fort with many casemates. Länsi Mustasaari is larger and had to be fortified with a more irregular trace. In the 19th century the Russians replaced the fortifications on Länsi Mustasaari with earthen mounds and longe-range artillery.


The little island of Särkkä, to the north-west of the other islands, was also fortified. It contains a small fort that could defend against an attack coming from the city of Helsinki (Särkkä is only about 500m from the mainland) and also barred the way into Helsinki harbour.

Relief map: the fort on Särkkä.
Särkkä viewed from Länsi Mustasaari. On the left, the hydrofoil 'Laura' begins its one-and-a-half-hour journey to Tallinn.

The Särkkä fortifications consist of a central double sided thin redoubt with a bastion at either end. To the south of the redoubt is a triangular battery and to the north there is a rather larger battery which is of more irregular form due to the shape of the island.

After The 18th Century...

Construction work on the fortress continued for some time after Ehrensvärd's death, but the fortress can be said to have been 'complete' by 1790. The fortress was considered impregnable by many and was thought to safeguard Helsinki against any attack.

In February 1808 the Russians invaded Finland and Hämeenlinna surrendered to them. The Swedish army under Johan Cronstedt retreated to Finland's western coast, leaving the fortresses of Suomenlinna and Svartholma isolated.

The ditch in the middle of the fort on Kustaanmiekka.

Svartholma fell in March and it was not long before the Russians entered Helsinki by land. With all of southern Finland under Russian control, Suomenlinna was completely cut off from support.

The inner fortifications on Susisaari.

The commander of the fortress, Carl Olof Cronstedt, surrendered to the Russians with hardly a shot being fired. He has been much-criticised for this move, but in reality he had been abandoned by the Swedish army and did not want to risk the lives of civilians in a pointless resistance.

under the Russians many new barrack buildings were constructed but the fortifications left unaltered. In the Crimean War British ships bombarded Suomenlinna for 47 hours. This caused much damage to the fortress and led to the Russians making large earthen mounds and emplacements for newer guns all down the western side of the islands. The damage was gradually repaired but the fortifications on Länsi Mustasaari were replaced with earthworks for more modern, longe-range artillery.

In 1906 Russian marines stationed at Suomenlinna revolted and government troops had to retake the fortress by heavy fighting. The mutiny was caused by social indifferences which later led to the Russian Revolution and the rise of communism.

The earthworks on Länsi Mustasaari.

Visiting Suomenlinna

A cannon in a casemate overlooking the Kustaanmiekka straights.

The fortress of Suomenlinna was transferred to the civil administration in 1973 and has undergone refurbishment and repairs since then. The site UNESCO world heritage list in 1991 and is now Helsinki's most important tourist attraction.

The fortifications are intact for the most part, except for those on Länsi Mustasaari, which were never repaired after the Crimean War bombardment. The island of Pikku Mustasaari houses a naval academy and the southern tip of Iso Mustasaari is a labour camp, so access to the fortifications there is limited.

Apart from these areas, access to the fortifications and the extensive tunnel systems is very good. They are also in very good condition, as lots of restoration work has been carried out on them recently. The fortress is extremely large, and it is definitely a full day out, even without visiting any of the 7 museums! There are several ferries to Suomenlinna, but the best place to start is the marketplace in Helsinki. Ferries leave from here about every half an hour, and the return trip costs about €15 and lands at the quay on the northern side of Iso Mustasaari.

View across the front of the hornwork on Iso Mustasaari. The labour camp is to the left.

The best place to head first is the visitor centre (open all year), if only to pick up a free map and orientate yourself. In the foyer there is a relief map of Suomenlinna as it was at the end of the 18th century, and beyond is a museum telling the history of the fortress (€5 entrance fee). Guided tours are available, but its much more fun to explore on your own!

For more info see:

Unlike the other islands, Särkkä is not linked to the rest of Suomenlinna by a bridge. It is now home to a sailing club and I believe it can be visited by taking a ferry from the southern tip of Helsinki. The fortifications there are intact.

Condition Access to fortifications Size of fortress Accessability of town Museum/Info Overall score
9 8 10 8 8 8.6
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