It was the Valar. Whatever theory you come up with has to accommodate the geography of the Hidden City of Gondolin (search Google Images to see it illustrated)

It was the Valar. Whatever theory you come up with has to accommodate the geography of the Hidden City of Gondolin (search Google Images to see it illustrated). Aule, Ulmo, or Melkor, someone fashioned it in such a way that it was able to preserve Turgon and the other elves for 400 years until it was betrayed to Morgoth.