The Wizard's Tower is a big, fortress-like building consisting of a center tower and 5 side-towers. It sits on a mysterious rock floating in mid-air at the south-western edge of Kessex Peak.
You can see the tower up close from Kessex Peak, and you can also see it clearly from the opposite side of the bay in the Fishermen's Haven and Stingray Strand.
It is currently unknown who the wizard was that the tower belongs to and what powers are hidden in the tower, but a villain named Galrath seeks access to the tower. In the quest The Villainy of Galrath you have to stop Galrath from gaining access to the tower. In addition, the Cult of Verata has based its operations by the tower, perhaps also to gain access to what is inside.
During the War in Kryta, Justiciar Marron attempted to finish what Galrath started by gaining access to the Wizard's Tower.