Following my post of yesterday here some old wooden locks used to keep the water level the same until closing of the Zuiderzee with the Afsluitdijk in 1932. No more low and high tide. But the water must refresh every night and the flow of river water in the Zuiderzee still let that level rise. So the water of the Zuiderzee will let out in the North sea by means of locks in the Afsluitdijk and the channel water in the Zuiderzee by a few modern locks remotely controlled. The gun is just for the tourists to show that this part of Amsterdam was once part of the harbor quarters but is not on its regular place. That was an old warship.