Hi Scot !

Is it an image you found somewhere and label yourself or is it a personnal work ? no judgment here but if you want to make it look like medieval, the base image is not so good for that I think. The picture is both too blurry (it looks like a satellite picture with a bad definition) and too "precise" (there's a lot of informations : colors of buildings, especially sharp layers of streets, some buildings or targets in the water...)

If you have some time before your game, you may redraw the city with just the data you need for your game. As medieval maps are, for obvious reasons, hand-drawn, you can only upgrade the feel of oldness ! A plungeon in dark coffee to finish the job and here you are with an old parchment !

And if you are, like me, a bad illustrator, here is my tip : when I want to make a map of something already existing, I put the image in a illustrator software (gimp, photoshop or even paint), display it exactly the size i want it IRL, reverse it horizontally, catch some transfer paper, and a few moments later, you have the basics silhouette of your city/country on a sheet of paper, ready to be inked or almost.

So basically my advice is: scrap it and do it again, witch is not really cheering but that's the best thing I can think of to match with your needs ! If you want to "medievalise" this very picture, I will be of no help, because I don't know a thing about digital art. Maybe a fellow cartographer will be of greater help !

Regards,
Skaald