Uspenski Cathedral.

Every brick in these walls has a war story. The 700,000 that form the cathedral were floated by barge from the ruins of Bomarsund Fortress in the Åland Islands, a military stronghold blown apart by British and French warships during the Crimean War in 1854. No plaque tells you this — run your hand along the rough, chalky brickwork at ground level on the quieter rear elevation, and you're touching siege rubble repurposed as a house of worship.

Above, thirteen onion domes clad in 24-carat gold leaf represent Christ and the twelve Apostles. The central dome rises 33 metres, roughly the height of an eleven-storey building, atop the highest rocky outcrop on the Katajanokka peninsula. Architect Aleksei Gornostayev modelled the design on 16th-century Russian tented-roof churches, and against Helsinki's white neoclassical waterfront the contrast is immediate — the one building on the skyline that belongs to a different architectural civilization entirely.

The temperature drops the moment you step through the door. Thick brick walls hold the cold even in July, and the air carries the accumulated scent of frankincense and candle wax from decades of Orthodox liturgy. This is an active parish, not a museum.

Pavel Siltsov's iconostasis dominates the far wall, its painted panels blending classical and Byzantine traditions to screen the sanctuary from the nave. Gold covers nearly every surface. But what most visitors photograph and then walk past is directly above them: stand at the centre of the nave floor and look straight up. The dome interior — stars painted against a receding heaven — produces a genuine vertigo, as if the building were taller on the inside than the outside.

Below the main hall, a crypt chapel dedicated to St. Alexander Hotovitzky sits almost always empty. He served as vicar of Helsinki from 1914 to 1917, was killed in Stalin's Great Purge, and canonized only in 1994. And somewhere on these walls, a gap marks where the Icon of St Nicholas the Wonderworker once hung — stolen in broad daylight in 2007, in front of hundreds of visitors, never recovered.

Before you leave, walk around to the back of the building. A commemorative plaque to Tsar Alexander II — who personally chose the cathedral's dedication to the Dormition of the Mother of God — faces away from the main approach. Tour groups never see it.

Then take the terrace steps for one of Helsinki's best free viewpoints: Senate Square's white neoclassical facades in one direction, the harbour and Suomenlinna ferries in the other. Seeing both cathedrals in a single frame — one white and severe, one red and gilded — tells you more about Finland's split cultural inheritance than any textbook.

From here, walk east into the streets immediately behind the cathedral: Luotsikatu, Merikatu, and the surrounding blocks of Katajanokka. This is one of northern Europe's finest concentrations of Art Nouveau architecture, locally called Jugend — ornate early-20th-century apartment buildings in a completely different register from the Byzantine cathedral at their doorstep. Most visitors photograph Uspenski and leave. The neighborhood deserves twenty minutes on its own.