In the 4th century, Antwerp was first named, having been settled by the Germanic Franks. About 980 Emperor Otto II built a castle (of which the main gate is still preserved and known as the Steen), as the centre of a mark or fortified border zone, which, with the adjacent Vismarkt (Fish Market), became the nucleus of the new city. Previously known as Antwerp Castle, Het Steen gained its current name in around 1520, after significant rebuilding under Charles V (by Domien de Waghemakere). The rebuilding led to it being known first as "'s Heeren Steen" (the King's Stone), and later simply as "Het Steen" (The Stone). It is now the National Maritime Museum.