This cover from Netherlands depicts 2 small stamps of same type, issued in 2015, celebrating the UNESCO World Heritage Site
Dutch Water Defence Lines. The defence line of Amsterdam was built between 1883 and 1920. The fortification is based on the principle of controlling the waters around a city. It contains a network of 45 armed forts and can temporarily flood polders extending 135 kilometres around Amsterdam. The site was originally listed as the Defence Line of Amsterdam in 1996. In 2021, it was expanded to include defence structures at a total of 9 locations and renamed as the Dutch Water Defence Lines.
