The Tower of Art.
The Tower of Art is an impressive structure widely believed to be the oldest building on Discworld. Standing at 800 feet and with 8,888 steps from the bottom to the top, it is part of the Unseen University in Ankh-Morpork.
It is completely without windows and, due to several repairs over the centuries, it has a gnarled appearance, much like an ancient yew tree. Atop it sit several small turrets and crenellations and within its stones entire ecosystems have formed, including species unique to the Tower of Art. A lack of predators willing to climb the vast heights of the tower has meant that several species of insects, birds and small mammals have been able to evolve fairly unaffected by the outside world, except by the magical waste from the university, causing some very odd results in the evolutionary pathways of these animals.
During the events of Sourcery, the Librarian used the Tower of Art as a makeshift library after Coin ordered the destruction of the previous library. It served as a base of operations for the Librarian and Rincewind's counter attack.
During Men at Arms, Dr. Cruces uses the Tower of Art as a vantage point, killing Acting Constable Cuddy and attempting to assassinate Havelock Vetinari.
When Cohen announced that Rincewind was to be the head wizard of the Agatean Empire, he briefly considered having the Tower of Art shipped to the city brick by brick as part of a rival University before abandoning the idea altogether.