This is a cheap and fast method to do it if you are in hurry or just want to get something done as possible. I am not calling this is cheating, I am just helping you save time as it comes to deadline or money.
1. For getting a scene you can take which photo you like, I usually take a wide photo with some buildings. This is a quick way to do is you already have all things done here. Best is to go out and take photos on buildings and stuff, collecting your own ref library is the best.
2.Now I usually create a blue channel for masking, and separate the sky and the ground for each layer. That way I can easily switch in between layer and work easier. I always start with the rough paint to see where the eyes point first in, it is easy the tower of course.
3.And now I am adding some details, now you can choose to add texture to it or you can paint some texture, I choose to paint because this is anyway a sketch. But it is good to add some extra texture for more details of course. In mattepaints the vantage point is extremely important. Without the point it is hard to catch the viewers interest. Believe me I have done a lot of mistakes too.
The last part I regret what I did so I delete the bottom part because I dont want the style to look like Dylan Cole hehe.