he North Sea and land below sea level
The Netherlands is known for reclaiming land from the sea through extensive land reclamation projects. Around half of the country's land is reclaimed from the sea, using systems of dikes, canals, and pumping stations to manage water levels and create new land.
ondergelopen grond
The dutch province of Flevoland was reclaimed from the 'Zuiderzee' (South Sea).
Growing crops
zeeland
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
In reclamation from the sea, you then have to protect the land with sea walls, expensive to build, needing maintenance.
The Dutch word for sea is "zee".