IJzen means to break the ice. During wartime, the ice in the moats around fortified towns had to be broken up, to prevent hostile forces from walking over the moat to take the town.
Sometimes, ijzen was the obligation of the citizens of the town, or of specific farmers in the area who rented the farm from the overlord. In other towns, the garrison was responsible for making sure the moat did not freeze shut.
French army crossing the river Waal, 1795. Credits: Joannes Bemme, collection Rijksmuseum (public domain)
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