The Repository of the Architects was a biome that could not be accessed through legitimate means. The entrance was behind a 5 BSC door at the Graveyard until its removal, which could not be opened due to the fact that the fifth Boss Stem Cell was not legitimately obtainable until the Rise of the Giant DLC, long after its removal.
It was merely a prototype for the Forgotten Sepulcher, and was not meant to be accessible under normal circumstances. If it were to be accessed through modding, however, the biome would contain many Cannibals, and an unopenable door.[1]
5 BSC spoiler