So I decided to go with the dotted borders, as Diamond and some others suggested. I adopted this style for the (smaller) borders between the different counties in both kingdoms - you'll see those as small yellow dots. I also changed the colour of the roads from red to black, so as to clash less with the overall greeish-blueish feel of the map. Lastly I added the names of the duchies, and that'll probably be the most visible change. I hope it's not too much. What do you think?

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PS: I added a fullsize image immediately, instead of a link to the picasaweb page in the image title. That should make navigation a bit easier.