A lateral light source means double the shading work but four times the realism. The top, front, left and right walls should have four different shades.
Apart from that, I don't understand the nature, shape and size of the building.
- Vertical grooves and frames, like in the front, normally mark prefab blocks in large building (contradicted by the continuous octagon at the top, which would be a huge and bizarre single-piece beam) or doors (contradicted by the lack of handles etc.) or vertically sliding doors (contradicted by the lack of an upper floor).
- If the top is a terrace, the octagon is far too thick as a protective waist-high wall, and there should be some access way.
- The vertical sides of the octagon have bands and lines that a smooth, flat concrete piece shouldn't have.
- Similarly, all 45° edges have disturbing knots and bumps that shouldn't exist (the main benefit of drawing 45° angles is that pixel lines are perfectly straight).
I think you should try a complete, meaningful building instead of a mere test of your well laid out tileset; you should adapt and compromise organic building designs to carve them into tiles, not design tiles before you make buildings with them.
EDIT: your embedded images shouldn't be links to the tinypic home page (completely useless), nor in fact links at all: this forum has a nifty javascript feature that enlarges the images on clicking, and turning images into links obviously disables it. If you have an useful link (for example to your site or to huge images), you can put it in text.