 In the 1550s, Tønder's port lost direct access to the sea due to dykes being built to the west of town at the direction of Duke Hans the Elder of Schleswig-Holstein-Haderslev, the son of Frederick I of Denmark.The town center is dominated by houses from the late 17th and early 18th century, when the town experienced rapid growth as a result of its lace industry.Prior to 1864, Tønder was situated in the Duchy of Schleswig, so its history is intertwined with the contentious history of Schleswig-Holstein.