Karazhan

The tower Medivh inhabited, near the Redridge Mountain on the continent of Azeroth. Existing long before Medivh inhabited it, it is unknown who these original builders were. It is also unknown who now resides within its walls.

It will eventually be an instance(but knowing Bliz, this may take awhile).

First description from the New York Times:


 * Back in the Karazhan design meeting, artists and developers were debating whether the horse heads' eyes should move to follow players exploring the area when Chris Metzen, Blizzard's vice president for creative development, poked his head in.


 * For 10 years, Mr. Metzen, 31, has been creating the rich stories and plotlines that have girded Blizzard's success. In his silver-rimmed sunglasses, shorts and hiking boots, he strode to the monitor as the others explained the concept of the horse heads.


 * "Well, I'm not so sure it makes sense just because it's a carriage house," Mr. Metzen said. "But check it out: in Warcraft 1 it said that Lothar was part of the Brotherhood of the Horse, and I thought that was kind of lame then, but maybe a horse head will work because of that. Yeah, that's hot."


 * Within a few minutes, Mr. Metzen determined that the history and culture of the tower's long-dead inhabitants decreed that the architectural animal motifs progress from horse heads at the tower's base to eagle heads a bit higher, culminating in lion heads at the grand opera house.


 * "But don't put these heads all over the place," Mr. Metzen told an artist. "Just sprinkling them in here and there will really sell the history to the players who are paying attention."