Rijeka is a Croatian port city on Kvarner Bay in the northern Adriatic Sea. It's known as a gateway to Croatia's islands. Korzo, the main promenade, is lined with Habsburg-era buildings. Nearby, the 19th-century Ivan pl. Zajc Croatian National Theatre has ceiling paintings by Gustav Klimt. The hilltop Trsat Castle complex, which includes a religious shrine, has sweeping views of the islands of Kvarner Bay.