Constructed as a part of the New Dutch Waterline, these four forts are today a national monument. They are built in half-moon shapes.
Wikipedia (Dutch) http://nl.wikipedia.org/wiki/Lunetten_op_de_Houtense_Vlakte
Address Koningsweg, 3585 Utrecht, Netherlands
Coordinates 52°4'10.704" N 5°9'3.96" E