The White Tower

The exact origins of the physical tower has been lost to history. The leading hypothesis is that it was a watchtower constructed by Skeggarian Dwarves  shortly afterAntecea's  fall, but the White Tower's significance does not truly come into play until the archwizard Christoph Heulen moved in.
Within the halls of this remote tower, Heulen conducted countless experiments and made untold strides in the arcane. It was here where he developed M.A.D (the Multidimensional Access Door), a network of interconnected doorways that allowed Heulen to access several major metropolitan hubs in but a few steps (and also how he was able to successfully run nearly a dozen "magic shops" as a front for his studies).
During the events of The Extinction War, the Tower served as the base of operations for the aptly named White Tower Alliance . MAP again allowed the physical location of the Tower to remain a protected secret, but unfortunately, the location was gleamed by spies within the Alliance and an army of Slaadi laid siege to the Tower in 1219 EP. The invasion force was repelled, but with its location compromised, the tower was abandoned in favor of housing the Alliance in Sigil.
Heulen returned to the tower only once to reclaim any of his remaining works that survived the siege and any subsequent looting. Since 1221, the tower has remained as a silent, empty ruin.