During a ride of 65, 120 or 170 km you will discover (a part of) the old defense line around Amsterdam, which was built to protect the capital in times of war. With an ingenious system, the land around the line could be inundated, which would make an enemy invasion of the capital impossible. An example of Dutch hydraulic engineering ingenuity, which has been on the Unesco World Heritage list for more than 25 years.