This is one of my older pieces. My first try to create an 'in city' environment. I used terragen (which still isn't in the software list here on cgtalk) to create myself a base to work on. See here for one of the, downsized, base renders (worked with multiple passes):

All the other stuff was added with help of photoshop. Original res 5000x3333x300dpi. Countless hrs of work but learned a lot.