Baba Vida Fortress

At the northern end of the park lining the Danube, the marvellously intact Baba Vida Fortress is a 17th-century Turkish upgrade of 10th-century Bulgarian fortifications, which in turn were built upon the ruins of the 3rd-century Roman fort of Bononia. There's little to see inside, but it's an atmospheric place. Watch out for uncovered holes and drops from the top.