After turning a set of AE layers in 3D layers, an immersive 3D environment can be built by using the flat elements as ground plane, walls, and roofs. Creative pre-production in Photoshop along with careful lighting and camera choices in AE can overcome the limitation of working with flat shapes that have no inherent depth.
To overcome the small production window into 3D space that typical comp settings offer by default, choose for your initial comp setting the largest pixel count and aspect ratio your system can handle without unwieldly redraw times. 2k and 4k resolutions can offer a wide window into your scene so that elements can be positioned and aligned correctly. Introducing cameras into the scene as production tools enables the extension of the backlot far outside of the "Front" and "Custom" viewpoints. Once the various elements are positioned, nest the large comp into a smaller one that will be your final render window into your project.
Creative use of lighting turns this scene into a decaying urban night scene. Ambient and Parallel lights establish overall mood while Spotlights create areas of interest and color. The overall dark atmosphere of this setting allows a wider range of Camera movement - which otherwise must be resticted to avoid revealing the 2.5D nature of the "3D elements." Mouse over scene to compare this 4k "window" with an HDTV resolution setting.
Here is the 10 second sequence of the scene from this viewpoint down to street level.