The First Men
Roughly 12,000 years before the reign of Robert Baratheon, the First Men came to Westeros on a land bridge between the eastern continent and the land that would later be named Dorne. These men brought bronze weapons and horses with them.
When they first arrived, they cut down the weirwoods in order to clear land for farms and holdfasts. This angered the Children of the Forest, who responded with violence. The First Men came to believe that the children could watch them through the weirwoods, and so chopped down those trees for safety and retribution.
From the Children’s Tower in Moat Cailin, stories say the children used powerful magics to destroy the land bridge, hoping in vain to prevent others from coming across. The remains of this bridge form the Broken Arm of Dorne and the Stepstones. In the end, though, the children only forced future invaders to turn to sailing ships.
The First Men spread throughout the continent, and even settled the Iron Islands. It was they who found the Seastone Chair standing on the shore of what would become the isle of Old Wyk.