From A Wiki of Ice and Fire
The Hightower is a castle and lighthouse in the port of Oldtown atop Battle Island. It is the seat of House Hightower, one of the most powerful families of the Reach.
According to a semi-canon source, the Hightower stands in the center of Oldtown, the city having grown around it. It is a massive stepped tower with a beacon on top to guide ships into port. It is one of the tallest structures in the Seven Kingdoms. It is speculated by fans that it is one of the nine Wonders Made by Man written about by Lomas Longstrider.
Ted Nasmith's depiction of the Hightower
References and Notes