I just recently got into pixel art, I'm working on my first piece but I'm having problems with some of the shading.  If you'll not the corner facing towards the viewer need to appear to recede into the building as the corner on the sides do.  I searched but I couldn't find any good examples of this type of shading.  Any help?

http://img375.imageshack.us/my.php?image=skyscraperfa9.png