Fort aan de Drecht in Uithoorn

You're standing at Fort aan de Drecht, right in the middle of the polder near Uithoorn. It may look like a quiet green mound with a moat around it, but this was part of the Defence Line of Amsterdam, the great ring of forts around the capital. Its job was to guard the dam sluice in the Drecht river. With sluices like that, the land could be flooded so an enemy army would get stuck.