Svartholm fortress

The Svartholm fortress (Finnish: Svartholman merilinnoitus; Swedish: Svartholms fästning) was built between 1749 and 1764 outside Loviisa in Southern Finland by general Augustin Ehrensvärd. The fortress, which lies at the mouth of the Bay of Loviisa, along with the planned land fortress at Loviisa, would prevent invading Russian forces to enter Swedish domain in present-day Finland.
Of the six planned bastions, only two were ready when the Swedish-Prussian war (1757?1762) interrupted the works in 1757.
The sea fortress of Svartholma was not ready, but was in defensible condition. The fortress was still not ready in 1808, when the Finnish War erupted, and two third of the guns and mortars were available. It had however been noted already in the 1760-ies, that it was not possible to place as many guns on the fortress as would be required to repulse a big attack from the sea. It had also been noted that the southern part left much to wish for.

Citation from Wikipedia