A monster's been unleashed, and an undocumented one at that, with this post-processing caper. I decided to try many effects and filters using Lunapic and see what emerged. I can no longer remember what I used to achieve what image – although some are obvious: Warhol and Obama effects; grunge, emboss and black&white filters. All I know for nearly certain is that the original photo is the first one, and that grunge is not a bad filter for some parts of Warsaw.
For far more accomplished, and sometimes quite stunning, post-processing, have a look at HeyJude's bench series collection for June.