I sketched a few ideas for the rooftop.

My primary reference has all been from Leicester, some from the top of Dominos, some from the top of multistory carparks and most from the Mansfield Police Station roof.
Secondary reference is a mixture of internet saves and game screenshots, (Mirrors Edge was the biggest help).

 This is my 3D plan of the Rooftop level. I exported the floor and used it in UDK as reference to build my whitebox.

 It took me 7 tries to get to the stage where it was complete except for the vent system that ran underneath.

This screenshot was taken at the moment i decided to scrap the vent idea. I had spent so much time getting the measurement right and finding out how to make the character crouch so it would all be possible, when i was finally ready to test it properly, the character would not drop down. Instead he just hovered above the opening and It was too much. I had to just cut and run with the next part of the plan.

All of my main assets are build and unwrapped. I havnt had very many problems with building them. The only one i found a challenge was the lettering for the "medusas" sign. I could use the text tool in 3ds Max because it uses up so many tris, I tried to just target weld and connect a lot of the verts but it would have taken forever, instead used the text as accurate references for each letter and modelled them separately.