Access by their "surface-dweller" allies is very strictly controlled. There are rarely more than a handful of non-KT visitors at any given time, and those carefully watched and guarded. Meanwhile ships are not permitted to land, dock, or anchor directly off the main island - any dwarven or other ships must trade at the small South-West port island of Ko-Duar, the Eastern port island of Ko-Elduar, or to choose not to sail to the highlands at all, instead exchanging goods at the Illud/Elven port of Stadt Verbundete, the "City of Allies", in the shadow of Blue Mountain.
This was the site of the first Ytarran capital, with a land bridge to the mainland protected by Yttronium barriers. In the dying days of the Ytarran Empire, these barriers fell and the Ytarrans relocated their capital to what is now the Dwarven capital of Tarquazhan. The extent of Ytarran ruins or other ancient infrastructure is unknown, as the area has been under KT control ever since the Ytarrans died, but it's difficult to imagine that much of anything there could still lie unexplored.