Windmolens is the plural of Windmolen.
If you are interested in knowing what the singular (enkelvoud) or plural (meervoud) of a word is you can go to www.vandale.nl.
If you put in a word it gives you the singular (if it is de or het, and the plural (meervoud). As well as a definition in Dutch.
You can also translate words from English (Engels-Nederlands), or to English (Nederlands-Engels).
Windmills to keep the right water level in polders used to be very important in the Netherlands and parts of Belgium. For instance the 19 world heritage windmills near Kinderdijk are all polder windmills. Their function was taken over by steam pumping stations (stoomgemalen) and later electrical pumping stations (electrische gemalen). According to https://nl.wikipedia.org/wiki/Gemaal_(waterbouw) there currently are some 550 pumping stations in the Netherlands, which are still very important (and will keep on being very important) to keep our feet dry and water at the right levels in all polders.