According to myth, Westeros and Essos were once joined by a great land-bridge - known as the Arm of Dorne - across which the First Men invaded Westeros. The Children of the Forest destroyed the land-bridge with magic more than ten thousand years ago, leaving behind only the shattered islands as evidence of its passing. In the present day, the Stepstones mark the border between the Narrow Sea to the north and the Summer Sea to the south. The Free City of Tyrosh is located on the northeastern-most island in the chain.