Always referred to simply as "The Steen," this medieval fortress on the banks of the Scheldt dates from the early 13th century, making this Antwerp's oldest building, and still exhibits Romanesque traces. You can walk into the courtyard even when the museum is closed. The glowering fortress has served a number of purposes over the centuries. Today it houses the Nationaal Scheepvaartmuseum (National Maritime Museum). There's an extensive library on river navigation and almost every nautical subject, as well as exhibits about the development of the port and maritime history in general. The most eye-catching are models of old-time sailing ships, like those of the Belgian East India Company clippers. Next to the museum there's an interesting industrial archaeological division with the remains of the old Antwerp port; this division is open Easter to November 1.