Tor Anrok

The ancient tower of Tor Anrok was once the home and stronghold of the House of Lothain, a noble family of High Elves who could trace its bloodline back to Tiranoc in Ulthuan. Although the Elves abandoned the tower long ago, most of the structure stands to this day. Due to the clever Elven design, the tower is very difficult to see from the surrounding areas -- so much so that legend has it that Tor Anrok can only be seen one night a month, and even then, only when moonlight strikes it. Either these legends exaggerate or the Elven magics that once obscured the tower have daded, for the tower can indeed be seen by the sharp-eyed and by those who know where to look for it. The tower itself provides an excellent defensive position. It is difficult to see and therefore attack; however, it affords an outstanding vantage point from which to view the surrounding lands.

Source
pg. 55
 * Warhammer: General's Compendium (supplement)