What could help you with the Z-axis is to try to match the lighting seen on the matte. If most of the shadows match but a few are off that may clue you in as to if you have an element misplaced. I almost guarantee you that the shadow of that lone tower is not going to match, given where you've placed it.
I don't want to hijack the discussion, but it did make me wonder: How did Whitlock work? Did he build a scale model of the subject, in this case specifically the fortress? Would he have used real-world structures (or photographs of them) as references for parts or sections of the whole scene? Did he create preliminary diagrams to show the overhead layout of the scene, or to plan perspective, etc.? If any of such stuff existed, it seems like that could be important to the project.