To keep the North Sea from overflowing the countryside during a storm
he North Sea and land below sea level
The Netherlands is known as the lowlands because a large part of the country is located below sea level. The Dutch have created an extensive system of dikes and canals to protect their land from flooding and to reclaim land from the sea.
ondergelopen grond
The country that is mostly below sea level is the Netherlands. About 26% of the country's land is below sea level, with its lowest point reaching 22 feet below sea level. The Dutch have built an extensive system of dams, levees, and pumps to protect their land from flooding.
The dutch province of Flevoland was reclaimed from the 'Zuiderzee' (South Sea).
The Dutch call their area of land reclaimed from the sea "polders." These are low-lying lands enclosed by dikes where water is drained to create arable land. Polders are a notable feature of the Dutch landscape and have helped expand their habitable territory.
zeeland
Growing crops
These walls are called dykes or dams. (dijken in Dutch)
It is called Flevoland, the 12th province.
I guess it would be called droogleggen/droogmalen "Inpoldering" would be another term.
They had sea walls and land walls to protect invaders from entering