The red-brick Russian Orthodox cathedral on Katajanokka, immediately east of the central harbour. Built between 1862 and 1868 to a design by Aleksey Gornostayev, with thirteen golden onion domes representing Christ and the twelve apostles. The interior — gilded iconostasis, frescoes, the smell of beeswax candles — feels worlds away from the white Lutheran restraint over on Senate Square.
Free to enter outside service hours. Combine with a walk along the Katajanokka shore, where ice-breakers moor in winter, and the art nouveau apartment buildings of the Katajanokka district.