Summerhall is a ruined castle in the Stormlands, near the borders of both the Reach and Dorne. It was formerly the summer palace of House Targaryen and the third and newest of the strongholds of that house, after Dragonstone and the Red Keep of King's Landing.
Summerhall was destroyed by fire approximately forty years before the events of the series begin. The fire killed both Aegon V Targaryen, his heir, and his lifelong friend Ser Duncan the Tall. It is rumored to have been caused by an attempt to hatch dragon eggs.
In the books
In the A Song of Ice and Fire novels, Summerhall has special significance for several members of House Targaryen. King Daeron II built it as a place of peace and relaxation away from court. One of his sons, Prince Maekar, adopted it as his main household. Along with King Aegon V, and his heir Prince Duncan the Small many members of the Targaryen household were killed during the fire. The deaths resulted in King Jaehaerys II taking the Iron Throne. However, the fire was said to have traumatised King Jaehaerys, and he died young only two years later, leaving his son Aerys II Targaryen as King. Summerhall was also the place, where a midget woods witch, who had prophesied that "a promised prince" would be born of Aerys and Rhaella Targaryen. Ser Duncan the Tall was likely killed in the fire as well, since Ser Gerold Hightower was Lord Commander of the Kingsguard the following year when Barristan Selmy joined.
Aerys II's own son, Prince Rhaegar Targaryen, was fascinated by the ruins of Summerhall. He was born on the day that the fire took place and visited it several times.