Some more technical drawing here, as we move on to the arches on top of all those openings you just blocked out. If you're doing this on the computer, there are a few techniques you can use to achieve these curves. I used the pen tool, but I could have also used the shape tool. Remember, you can always drop in an ellipse and erase the lower half to create an arch. If you're drawing by hand, you can either use a plastic stencil, or just try to have a steady hand. With pen and ink, I would lay down even more guide lines and then draw the curves freehand.
Go ahead and draw the inner part of the arches. This is where you really get a sense of depth in the layers of the structure. For the bottom of the archways, you're going to be using one-point perspective. To keep it simple, just remember that all receding lines go toward the center of the image. For a view such as this, that's enough to do the trick. You could go crazy laying perspective lines all over the place, but as long as you keep checking your reference and keep that center point in mind, you'll be ok. For more complex camera angles, however, a knowledge of proper perspective will be absolutely necessary.