PJ Image Specs
It is the contrast. The image displays a wall (which is a symbol of a civilized nation) next to an eagle (as you say, it portraits Rome) but the image in its totality seems depressing. It is not all gold and shine and marble but ruins, dark and sunset.
While I like the piece overall, and I can see how the eagle relates to the Roman empire, I don't understand the rest of the image and how it depicts the fall of the empire.