Svartholm fortress

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The Svartholm fortress (Svartholman merilinnoitus, Svartholms fästning) was built between 1749 and 1764 outside Loviisa in Southern Finland by Augustin Ehrensvärd. The fortress, which lies at the mouth of the Bay of Loviisa, along with the planned land fortress at Loviisa, would have prevented invading Russian forces from entering what was then Swedish territory in present-day Finland.

Adapted from Wikipedia (CC BY-SA). Photography via Wikimedia Commons.