Overwaard Molen (Windmill) No.4 to No. 7 at Kinderdijk. Built in 1738 and 1740, a group of 19 windmills were built to keep water out of the Alblasserwaard polder in the province of South Holland, Netherlands. It is the largest concentration of old windmills in the Netherlands. The 9 Overwaard windmill are built with woods. The Kinderdijk windmills have been a UNESCO World Heritage Site since 1997.