Fort Alexander — universally known as the Plague Fort — is one of the most atmospheric sites in the St Petersburg area: a circular brick fortress built on an artificial island in the middle of the Gulf of Finland in 1845. It was never seriously tested in battle; its role shifted from coastal defence to plague research laboratory, and then to complete abandonment. Today it sits silent in the water, its brick walls weathered, its circular galleries open to the sky.
The fort is accessible by boat from Kronstadt during summer, with regular tour trips organised from the city waterfront. The interior is in a controlled state of picturesque ruin — the walls and vaulted casemates are intact, but nature has colonised the courtyards and upper levels. The effect is genuinely striking: a Victorian military-industrial building reclaimed by grass and moss, with views across the grey Gulf in every direction. The combination of architectural interest and dark history makes this one of the most memorable excursions from St Petersburg.