Hey Ceraus,
Nice maps and a great idea.
I popped them both into PS to see what happens if you try to line up the two maps.
With some tweaking you could make the sewers and and the city line up. Though maybe you have plans to remake the city map?
I'm no expert so this is more of a question but can they/did they tunnel under rivers?
I think making both maps line up, knowing all the entrances and exits would make for a great base for writing some great adventures.