I would use maps for this.
What I mean is, I would use satellite data or a hand made map in PS to get me very close, then I would use the normal TG tools to get me the rest of the way.
If it were my project I would use satilite date from some place in Asia that look similar. Than I would ether alter that map or use it as is. Once loaded in TG I would either leave the data as it aperes or cause it to be stretched and scaled until it looked like I wanted, then I would add fractals and colors until I was happy
Also, Its a good idea to post a test render with your inspiration image so we know where you are in learning, and can give you more that general responses as I just did.
Lastly, don't expect to get "Exactly" a reproduction of the painting. But "very close" is more than possible. Just some position and heights of the forms will be different.
For adding the rivers you can again use PS to draw a path, and then displace that with negative values to create the river beds. But with this simple way, you will end up with some parts of the river being very shallow or very deep. So you want to make sure that the base of the towers is very flat(ish) to begin with since you will add the river map to the terrain map(s).
