Hi,
After some days of erring way, I post this trial as a work in progress. I know it can largely be improved, but this is what I manage to do with my little knowledge of GIMP...
The beautiful flashy green and magenta are just there to delimit the future walls and the river's bank.
I am not really satisfied by the paved roads pattern, but I didn't manage to get one that comes into roads alignment... maybe with a rotating brush ?

