Board Thread:General Discussion/@comment-92.233.139.244-20130530214619/@comment-9887604-20130626150227

Once again I will quote from Before the Ages of Men

"When Magic (Magnus), architect of the plans for the mortal world, decided to terminate the project, the Gods convened at the Adamantine Tower [Direnni Tower, the oldest known structure in Tamriel] and decided what to do. Most left when Magic did. Others sacrificed themselves into other forms so that they might Stay (the Ehlnofey). Lorkhan was condemned by the Gods to exile in the mortal realms, and his heart was torn out and cast from the Tower. Where it landed, a Volcano formed. With Magic (in the Mythic Sense) gone, the Cosmos stabilized. Elven history, finally linear, began (ME2500)" Before the Ages of Men

Adamantine tower was not directly tied to creation, but more to the ordering of time. When Magnus left time was finally able to become linear. Adamantine tower was also where the Aedra tore apart Lorkhans body, and Auriel shot Lorkhans heart into the sea.

So I take this statement that the Adamantine tower was the oldest structure known to the mortal races, so by that logic there may have been structures unknown the the mortal races.