Just east of Amsterdam, where the city's suburbs give way to water and polders, a narrow strip of land runs out along the old shore of the IJ and the IJmeer toward Muiden. The Diemerdam is an old dike, and the works along the IJ before it are part of the Dutch Water Defence Lines, the system of forts, dikes, sluices and inundation fields that the Dutch built over centuries to defend their cities by flooding the land around them. The Defence Line of Amsterdam, a ring of forts around the capital, was listed by UNESCO in 1996 and later joined with the New Dutch Waterline into a single site.
The idea was ingenious: in case of attack, the defenders would open sluices and flood the polders to a depth of about forty centimetres, too deep for soldiers to wade through and too shallow for boats. Forts guarded the dikes, roads and railways that crossed the flooded areas. The Diemerdam had a fort and batteries, and you can still see earthworks and bunkers among the greenery. Out in the IJmeer, the island fortress of Pampus, reachable by boat from Muiden, is the most spectacular of the forts.
The area is a popular recreational zone, with paths for walking and cycling, beaches and wetlands full of birds. The views over the water, with sailboats and the distant skyline of Amsterdam, are beautiful, especially at sunset.
As a rider, you will find the roads flat, straight and busy, with motorways, bridges and urban traffic all around. The dike roads are narrow and often reserved for bicycles and local traffic, so park and walk if you want to explore the defences. Wind is the main challenge, blowing hard across the open water and polders. Muiden, about four kilometres away, is a charming small town with a medieval castle, the Muiderslot, where the poet P.C. Hooft entertained artists and scholars in the seventeenth century, and a harbour full of yachts.
Diemen has fuel, shops and services, and Muiden has cafés and restaurants by the harbour where you can enjoy fresh fish or apple pie with a view of the castle. For a longer ride, follow the dikes along the IJmeer and the Gooimeer toward Naarden, a star-shaped fortress town that is another highlight of the waterline.