St Isaac's Cathedral dominates the south bank of the Neva with its enormous gilded dome — 101 metres high, the fourth largest in the world. Inside, the interior is suitably overwhelming: marble columns, malachite and lapis lazuli pillars at the iconostasis, mosaics that took half a century to lay, and a soaring central drum painted with religious frescoes.
The real draw, though, is the colonnade walk around the base of the dome. A 262-step climb up tight spiral staircases delivers you to one of the best panoramas in Russia — the Neva spread out below, the spires of the Admiralty and Peter and Paul Fortress framing the river, and on a clear day the Gulf of Finland glinting on the western horizon. Like the Church of the Spilled Blood, St Isaac's is officially a museum rather than an active church, though services are occasionally held.