What if just below the Last City there was a whole subterranean complex of ruins which used to be the previous city before the Golden Age. And what if guardians could access these underground ruins where they could find old decaying architecture, artifacts and records that explained past civilizations of humanity, the Golden Age and the fall. It would be kind of like when you would find terminals in Halo 4 or finding pieces of records like you would do in ODST. Exploring the vast ruins and piecing together what had happened.
Sounds like futurama with the city under another city