Azshara Tower is located at Bear's Head in Azshara. The only way to get to it is to receive a teleport from Sanath Lim-yo. [28.1, 50.0] It is the home of Archmage Xylem, Joy Ar'nareth, and Rayan Dawnrisen, and also the shelter of Coral Moongale. It is not a proper subzone, and is only named by the return teleport spell given by Nyrill.
After Cataclysm, Archmage Xylem is presumed dead, he appears in game only as Image of Archmage Xylem and the Azshara tower is now called Xylem's Retreat, sometimes Xylem's Tower. Inside the building at the top floor, there are many Images of Xylem running around doing different tasks.