The Northern Forts are a series of coastal fortifications built on the tsarist orders at the turn of the 20th century to protect the Karosta naval base from seaborne attack. They were used for roughly twenty years, then abandoned when naval warfare changed beyond their design parameters. Since then, the sea has been slowly eating them.
The result is one of the more eerie landscapes on the Baltic coast: massive concrete casements and gun emplacements, partially submerged or perched at strange angles where the shoreline has eroded beneath them. The ruins are overgrown with grass and graffiti, and the atmosphere is unambiguously post-apocalyptic. Getting there requires walking through Karosta and then along the beach north of the breakwater. The ground can be unstable near the water — don't climb on structures that look precarious, which is most of them. Visit in the grey light of an overcast day for maximum effect.