Adamantine Tower

The Adamantine Tower, also known as the Direnni Tower, Ada-Mantia or the Ur-Tower, is the oldest known structure on Tamriel and perhaps the entirety of Nirn. Akatosh built it around the Dawn Era for the meeting known as the Convention, where the Aedra decided what they would do with Lorkhan. It is located in High Rock, on the Isle of Balfiera.

The Adamantine Tower was the first Tower to ever be created, it's stone being the Convention itself.

The Convention
The Convention was an event that ended the Dawn Era and started the Merethic Era when Akatosh built it in order to discuss the punishment of Lorkhan after his treachery in tricking the Aedra into creating Mundus, the Mortal Plane. They decided to remove his divine spark, in this case his Heart. In the war against Lorkhan, hero-god Trinimac managed to remove his Heart and tried, with Akatosh, to destroy it. However, the Heart was tied to the existence of Nirn and therefore couldn't be destroyed. Auri-El (Akatosh) fired it from his Bow somewhere into the east, where it landed in the sea, forming the island of Vvardenfell and the Red Mountain, a gigantic mountain that dominated Vvardenfell.

Direnni Clan
Sometime after these events, the Direnni clan decided to use it for their operations. Many Dirennis were well-known, and the Tower was later renamed the Direnni Tower, in their honor.

The Curse
Some time prior to 3E 405, Lysandus's mother, Nulfaga, put a curse on the Tower to trap Medora Direnni. After the Tower was cured by the Hero of Daggerfall, the Hero saved Medora and restored the Tower to its original state.