The main hall, where sat the throne of Thranduil and where he questioned the Dwarves was enterable through the front gate and was very high and columned. Other caves were smaller and probably included various living quarters, storehouses and butler's camber. There were also various prison cells which at one time held Thorin and the Dwarves. As a river ran under the caves, there was a barrelroom and a portcullis which the elves used to float their empty barrels of Wine and foodstuffs.
It is not known how old the cave system is but it may be older than Oropher's time. After his death, the realm came under the rule of his son Thranduil who would rule from then on. As the forest became dark and dangerous the halls became secretive and well protected dwellings.
In TA 2941, Thorin and the Dwarves were imprisoned here in the underground cells.