Around 1400 was Noordholland an area between the North Sea and the South Sea (now IJssellake) with many lakes. During the Middle Ages Holland was one of the most important trading countries and many traders earned a lot of money. Since even in that period land was scarce in Holland they decided to finance the reclaiming of land by means of windmills. Successively many "polders" were created: a.o. the Purmer, the Beemster, the Wormer and maybe most famous the Schermer. If you compare the present map with the (reconstructed) map of 1388, you can see the enormous developments that have been made in our area. The darker part of the map represents the present situation.
Due to the difference in height between the water in the polder and outside (often 7-8 meters = 8 - 9 yards) mills were used at three different levels. Regretfully many mills were taken down in the thirties, although there are still more than 20 mills within 10 km of Ursem. If you are interested to know more about this, give me an email and tell me what you're specially interested in. I will try to answer your questions.