Very little is known about this Era, it was far before the robots and current humans inhabited the Sentai Mountain, and before it was split. We get a small glimpse of this Era in Exo-Force online comic 26, and Exo-Force Book 4: Ghost of the Past. We know that during this era, some experiments were done and created the creature known as Nightmare 3. Also during this time all the battle machines discovered in the golden city were built, and were used to defend the city during that era as well.
Joev14 tries his best to fill in information about this era and clear up mysteries of this time in his fanfic Book 2: Golden City Origins