Romans visited Friesland, in northern Netherlands, around 50 B.C. (They didn't stay there, finding their ultimate border at the great rivers, the branches of the mighty Rhine splitting on its course westward.)
Roman historians wrote that at that time, people in Friesland were living artificial mounds today known as "terpen."*
The terrain, Northern European delta soil, was (and is) near mean sea level.
In its natural state, much of the land was only dry during the ebb-tide and then only relatively so.
These terpen, constructed to various heights up to ten meters above the ground, were sometimes extensive in area one was greater than 37 acres [15 hectares.] Their production was vigorous in the region, for a period.
Archeology finds that after AD 1000, the residents of Friesland stopped building terpen.
This was because the area had been surrounded by dikes, and was dry.
* Terpen is the Dutch-language plural of "terp," the Frisian cognate of "dorp," the Dutch word for "village."