The walls of Amsterdam were built in the Middle Ages to protect the city against attack. The Medieval walls were replaced with a series of bastions in the 17th century. In the 19th century, the walls were torn down and replaced with the Defence Line of Amsterdam (Stelling van Amsterdam), a fortification line which encircled Amsterdam at a distance from the city. Remains of the Medieval and 17th-century defenses include the former city gates Waag and Munttoren, as well as the defensive tower Schreierstoren and the watchtower Montelbaanstoren.