Fortification is usually divided into two branches, namely permanent fortification and field fortification. Permanent fortifications are erected at leisure, with all the resources that a state can supply of constructive and mechanical skill, and are built of enduring materials. In Holland other cities with fortifications: Heusden, Willemsstad, Gorinchem, Naarden, Zaltbommel, Enkhuizen, Harlingen, Hulst, Geertruidenberg, Hellevoetssluis, Ravenstijn, Nieuwpoort, Woudrichem, Loevenstijn, Zwolle, Bourtange.