Board Thread:General Discussion/@comment-92.233.139.244-20130530214619/@comment-9887604-20130626144010

Well the Adamantine Tower was constructed in ME 2500, also the end of the dawn era. Time gets real fuzzy around that particular time. However I am not sure if it is anecdotal that the Adamntine tower is the oldest physical structure on Nirn.

"According to King Harald's bards, ME2500 was the date of construction of the Adamantine Tower on Balfiera Island in High Rock, the oldest known structure of Tamriel. (This corresponds roughly to the earliest historical dates given in various unpublished Elvish chronicles.)" Before the Ages of Men

So it just confirmed to be the oldest known structure, there may be other structures unknown possibly.