As far as I know, we don't have fortifications like the ones already shown (and if we do, I don't know of any), so I will show you a medieval fortification. I show you Slot Loevestein. This castle was part of the Nieuwe Hollandse Waterlinie untill the second world war. It was also used as a state prison to keep Hugo de Groot locked up. For those who don't know, Hugo de Groot is one of the people who laid the foundations for international law. It's a 1 hour drive from my parents' house.
The Netherlands have a long history of fortifying their cities. The best preserved can be found around Naarden, Hulst and Heusden. A lot of pictures: http://www.google.nl/search?hl=nl&gs_rn=1&gs_ri=hp&#...
Ah, I totally forgot. Many of the older cities started out as fortified towns, but many cities don't have their city walls anymore. I also totally forgot about castle Heeswijk, which is even closer to my parents.