The Summerwall, along with the Lake of Lost Memory, forms the northern border separating Gristamere from Summerlund. After Gristameri troops were pushed back out of Summerlund’s northern border during the Battle of Seowyn’s Crossing, royal troops began construction of a wooden pallisade wall to mark the borders and repel further incursions by the goblins. Once the Second Witching War staggered to a conclusion, construction on a more permanent structure began, and a vast stone wall was built, partly by normal means and partly through ritual and artifice.
This wall is part of the responsibility of the Duke or Duchess of Ostralm, as it falls within their borders. To date, while there have been minor attempts to break the wall, they have all failed. The goblins instead must sail over the Lake of Lost Memory (which generally hasn’t gone too well) or sail around through the Holds of the Sea Barons (which hasn’t gone too well either).