Well, in A Storm of Swords it was mentioned by Catelyn that something momentous happened there--Barristan slew the last of the Blackfyre pretenders, Maelys the Monstrous on the Stony Steps at Summerhall. Also, when there is a mention during the discussion of raising the stone dragon, one of the Florents asked if they had "learnt nothing from Summerhall". It's a very intriguing mention, and since Dunk and Egg were there, I was wondering if we would ever see that written into a story...it sounds very intriguing. I presume that the events must revolve around Aegon V, Ser Duncan the Tall and the last of the Blackfyre pretenders.
You're confusing two different events.
Ser Barristan slew Maelys on the Stepstones, the island chain where the War of the Ninepenny Kings was fought. It had nothing to do with Summerhall.