Its very good. I like it.
I think the city wall distracts me a little from the city itself - I think I expect them to be stylized into a regular pattern.
The piece makes me wonder how truly flat your cities are and if notable buildings\city features might not be artificially enlarged just to point them out. (Stylized once more)
It looks good.
Here's a side view of Bologna (from a thread at Enworld). Its not really laid out flat - it sort of rises at about 12 degrees to show more city. I'm sure the vertical scale is proportionate to importance not actual height.
Your walls give the city a mysterious quality - I don't know what they're hiding.
And a link to a cool website of Italian Maps:
Its in Italian but the pictures are good.
There are a number of photoshop parchment tutorials around.