terpen

…built the first seaworks—dikes and terpen (or werden), mounds to which they retreated during periods of high water. The volume of these terpen ranks them among the great engineering works of humankind.

…the inhabitants lived mostly on terpen, or man-made mounds. Slowly the Frisians brought the lower-lying land under cultivation and protected themselves against the sea by building more terpen (dikes were not practicable). Most of these were in the modern provinces of Friesland and Groningen; the east bank of the Rhine…