Phew! That makes my suggestion a lot easier
The idea was to create a new perfectly square and perfectly symmetrical brush (identical no matter which side you have at the top) using bits and pieces of the border, or even just a simple pattern of circles or a blank bevelled square. Then all you would have to do is use the colour picker to pick the colours from the existing border, and dab it once in each of the four corners.
A lot of mappers use all kinds of corner designs - some elaborate and involving miniature scenes in little circular or quarter circle cameos and such like, but I don't think you really need to go that far unless you have a fancy to do it like that. Simple is just as good and effective
For future reference you might also want to make a brush out of that frame design to go with it.
Well that was my idea. Maybe you have an even better one of your own
EDIT: You just ninjad me! LOL! I like the lighter one because the darker one merges too much with the sea. You might find a subtle line of a very much darker blue around the inside edge of the frame will help to separate it just a bit more - whichever one you decide to use.
I think the roads tie the houses together a bit better - define the city.