I am still working at 25% of the viewable size. I ended up erasing the second row of windows because they were a bit too high. I am also starting to put in the fire escape. Most of the time I am using a round brush for the fire escape, rails, pipes, etc. One thing to always keep in mind when working on an illustration is edge control. Try and keep a good balance of hard and soft edges.
Don't forget about the little things that will make an illustration more interesting, like little spots of light. Also adding spots of light in dark areas helps to break up the space.