The original plan of the bastion fortress was strongly influenced by the ideas of Vauban, a Frenchman and the foremost military engineer of the time, and the principles of the star fort style of fortification, albeit adapted to a group of rocky islands.

The islands were then used as a base for the Russian Baltic Fleet in World War I, with Russia beginning the construction of the Krepost Sveaborg in 1915.

This island, which has the greatest concentration of fortifications was renamed Gustavssvärd (King Gustav's sword) (fi: Kustaanmiekka) during the construction by Sweden.

[3] The lack of coastal defenses was keenly felt with Russian landings in Helsingfors in the spring of 1713 and the Swedish failure to blockade the Hanko Peninsula in 1714.

A Russian naval campaign against the Swedish coast towards the end of the Great Northern War further outlined the need to develop Finnish coastal defenses.

Immediately after the war ended the first plans were set in motion in Sweden to construct an archipelago fleet and a base of operations for it in Finland.

After lengthy debate, the Swedish parliament decided in 1747 to both fortify the Russian frontier and establish a naval base at Helsingfors as a counter to Kronstadt.

There were two main aspects to Ehrensvärd's design for Sveaborg: a series of independent fortifications across several linked islands and, at the very heart of the complex, a navy dockyard.

In addition to the island fortress itself, seafacing fortifications on the mainland would ensure that an enemy could not acquire a beach-head from which to stage attacks on the sea fort.

Initially the soldiers were housed in the vaults of the fortifications, while the officers had specially built quarters integrated into the baroque cityscape composition of the overall plan.

The most ambitious plan was left only half completed: a baroque square on Iso Mustasaari partly based on the model of Place Vendôme in Paris.

Due to repeated Russian threats in 1749 and 1750, more effort was placed on the island fortifications at the expense of those on the mainland, so that a safe base of operations could be secured for the Swedish naval units along the Finnish coast.

[7] This period in Swedish history was known as the Age of Liberty, during which the kingdom was under increased parliamentary control, divided into two political parties, the Hats and the Caps.

Efforts to improve the fortress continued under Jacob Magnus Sprengtporten, but his tenure was cut short by disagreements with King Gustav III.

Once again efforts slowed down as garrisons were reduced, and in 1776 Sveaborg's commander reported that he could not even man one-tenth of the artillery placed in the fort.

By cutting the coastal sea route past Hangö, Russians prevented supplies from being shipped from Sweden to Sveaborg.

The Swedish fleet finally managed to set sail for its base at Karlskrona on 20 November when the Baltic Sea had already frozen severely enough that ice had to be sawed open before some ships could move.

The reasons for Cronstedt's actions remain somewhat unclear; but the hopeless situation, psychological warfare by the Russians, some (possibly) bribed advisors, fear for the lives of a large civilian population, lack of gunpowder, and their physical isolation are some likely causes for the surrender.

After taking over the fortress, the Russians started an extensive building program, mostly extra barracks, and extended the dockyard and reinforced the fortification lines.

The bombardment of Sveaborg (also known then as Viapori) by the forces of Richard Saunders Dundas and Charles Pénaud on 9–10 August 1855 lasted 47 hours and the fortress was badly damaged, but they were unable to knock out the Russian guns.

The fortress and its surrounding islands became part of "Peter the Great's naval fortification" designed to safeguard the capital, Saint Petersburg.

There is a minimum-security penal labor colony (Finnish: työsiirtola) in Suomenlinna, whose inmates work on the maintenance and reconstruction of the fortifications.

For the general public, Suomenlinna is served by ferries all year, and a service tunnel supplying heating, water and electricity was built in 1982.

Between 2 and 6 September 2015, the Finnish postal service ran a test of the use of drones to deliver parcels between Helsinki and Suomenlinna.

It includes the following two verses about the fortress, which allude specifically to the "Gustav's Sword" (Gustavssvärd) bastion and its guns: It looks out over sea and fjord, With eyes of granite.

[21] Sveaborg is also mentioned in the lyrics of Gunnar Wennerberg's 1849 hymn O Gud, som styrer folkens öden ("Oh God, who steers the people's fate"), in which Sweden's "age-old freedom" is described as being "...our protection in gloomy dangers, our consolation in every bleached sorrow, our defence against the hegemon's forces, and stronger than Sveaborg".

[22] As these words were written forty years after the Swedish cession of Finland to Russia, the use of Sveaborg as a simile has deliberately historic overtones.