Interesting concept. I'd like to see the city itself and also the dykes in future versions. What confuses me on the map a bit is the moat in that the dykes (or rather, what I assume to be the dykes) are open in both images. Or is the first image at low tide? If that's so, perhaps you should make three maps: the city and surroundings at low tide, the city and surroundings at high tide with the dykes closed and the city and surroundings at high tide with the dykes open.