The construction of the fortifications of the city of Willemstad, not far from my hometown, began in 1583 during the Dutch War of Independence (Eighty Years’ War) against Spain. The city in itself was not important, but more so its strategic position in the defense lines of the entrance from the North Sea as well as the big rivers.
The original layout was a star fort and all the bunkers were mostly or entirely covered with earth and grass. A good camouflage, because they were invisible from afar.
Through the centuries the bunkers have been amplified and they were used as military quarters till 1926. During WWII the German army put them to use as an arms deposit. Now it is a monument, although some maintenance work is badly needed.
My contribution to Cee’s Black & White: Hiding or Camouflaged