A vast 19th-century star-shaped military fortress commissioned by Tsar Alexander I, one of the largest and best-preserved fortifications in Eastern Europe. The complex of red-brick barracks, powder magazines, and earthen ramparts covers a wide area north of the city centre and retains an eerie, time-capsule quality — much of it is unrestored and quietly decaying, which adds to rather than detracts from the atmosphere. The fortress now houses the Mark Rothko Art Centre and a handful of other small institutions, but large sections remain open for free exploration. Allow a couple of hours to walk the ramparts and courtyards properly.