"The Empire" would refer to a specific political empire
"empire" could refer to that same entity, or could also refer to a not-real thing, but an idea.
For instance "That baron has dreams of empire motivating his territorial acquisitions"
So Ruins of Empire could refer to A. The ruins of The Anuirean Empire or B. the ruins of empires in general, or C. The ruins of the idea of "empire" (i.e. more de-centralization, worse roads, less trade, tougher to communicate, people are no longer Anuireans, but now Ghiestians or Rosoeneans or Avanese)