This is a very early version, but I thought I'd already share it with you guys, so you could show me the right direction. The city this is about is called Irriac, and can be found on the western shores of this map (a few dozens of miles north of Fyrnan). It lies in an area that generally lies lower than high tide, and is only protected from the sea by a series of dykes. When threatened by a siege, the inhabitants of Irriac can open the locks on the outer ditch during high tide in order to inundate the surroundings. The city itself as well as the island on the sea side have been built on higher ground, and will not be flooded.

What I'm trying to create is a map that shows the city under normal conditions, and another after the inundations.

Here is my first go. What do you think of it?

Click image for larger version. 

Name:	Irriac_normal.jpg 
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ID:	50259 Click image for larger version. 

Name:	Irriac_flooded.jpg 
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ID:	50260