Welcome to the Guild, Rathko, and have some rep for dropping a map in your first post

I know Milan quite well, but I've never seen it from this perspective before. I didn't even know it was a Roman city. Did you research the ancient layout/buildings extensively, or does this include some creative license?

I'm afraid I don't have much in the way of suggestions for you, because I'm not sure how you can get that 19th century lithographic feeling you want. A few things you might want to consider: try to make the map less sharp and less digital looking. If the look you're going for is a bit like this London map, then making the lines just a little bit less straight and adding just a little bit of colour or shading, can help distinguish between roads and farmland outside the city. Using a denser pattern of strokes on the building blocks might help them jump out a bit more, and that in turn will highlight the roads.

I think perhaps the key thing is to reconsider how you're doing the ditches (I assume that's a ditch, outside the walls and disappearing off the bottom and lower right edges of the map?) IF you make them more clearly ditches, you can use the same technique to indicate the boundaries between farms/fields

I hope some of that is helpful. You've got a very good start here, I'd say.

Wingshaw