Pampus is located in the former inland sea the Zuiderzee just a stone throw away from Amsterdam. In the 17th century the sailing cargo ships became bigger while the entrance to the port of Amsterdam became more shallow over time. At low tide it used to block the access of seagoing ships to the Amsterdam port. It was quite a handicap for the VOC, the Dutch East India Trading Company. In the late19th century an island fort was built at the site as one of a ring of 42 fortifications surrounding Amsterdam. The fort was abandoned in 1933 when the Zuiderzee was closed off from the North Sea by a dam and this body of water became the lake IJsselmeer.
The island is part of the the Defence Line of Amsterdam, a system of defense works constructed between 1880 and 1920. It consists of a ring of forts, dams, locks, dykes and ramparts around Amsterdam. By flooding the surrounding area, Amsterdam could, within 48 hours, be secured against an invasion. The system of 42 forts presented a formidable obstacle to any enemy trying to overtake the city. The Defence Line of Amsterdam was placed on the World Heritage List in 1996.