When I want my circles to be perfect I don't use selections, I use a brush. Start with something big and change the size accordingly then I just drop the brush size down and change color to black, drop down in size again and change to white...keep repeating until too small, I guess it has something to do with the way these programs handle the math behind the function that a circle ends up as a hexagon. Then I just delete the black. As for the bridges, it's kinda hard to get creative from top-down so just go pattern searching. The shadow is where you can give some hint of shape to your bridge, like arches and what not. This goes for the central tower as well, draw out a black shape that represents the side view of the object. Then skew, distort, blur etc.