This impressive riverside citadel is enjoying an ongoing renovation that's recreating its historic atmosphere. Look for the restored Nicholas Gate near the excellent Mark Rothko Art Centre. Built on the orders of tsar Alexander I on the eve of the Napoleonic wars, the fortress served as an imperial stronghold during two Polish insurrections in the 19th century and as a home away from home for tsars exploring their realm. It's a 3km walk northwest of the centre along the river.