Hi all,
It’s a steep and endless learning curve for all this! I use Twilight free edition too.
For me the bricks of the structure perhaps look a little flat. I tried to find the same brick texture as you but this was the closest I could get. Did you add a bump map to the textures to try to give the illusion of a bit of depth? (I know my paving scale is set too small but oh well…)
Somewhat a revelation for me a few months ago was exporting a 2d file of the shadows only, as a nice fast way to get correct shadows onto a background image. I would do the same as you to get the correct ambient occlusion type shadows on the model - ie. the green reflections in the shadows on the right pillar on the model - I couldn’t download your model, so I quickly made another and super-imposed it over a copy of your image, just to get the grass background… So, in Photoshop I paste in place (ctrl+shift+v) a shadows only image (same pixel dimensions as the render) and put it in Multiply mode to get rid of the white. Then you just play about/can stick it in whichever layer position you feel best to give the best look. Hope that makes sense - just though I’d share 
