Frostfound

"Towers and ramps and galleries and stairs of ice, raised and spun like an architect's honey-dream. No spider ever wove so complex a web. The towers are utterly pristine, untouched by human life, but a pitiable encampment squats by the dock."

Frostfound is quite literally an ice castle. Every year the people of Irem and Whither meet here for contests; they are trying to determine the difference between a question and a riddle. Those who prove themselves in the contests can enter the castle, which is sacred ground, but nobody has done so yet...

Trying to go beyond the Horizon will either warp a captain here or there, because there is only one way North.

And what of the secrets of the Gods?

Any explorer foolhardy enough to enter will likely find their mind slowly degrading as the frozen castle eats their memories and stories. Between the warped halls and deep crevices might lie the secrets of the gods... but let's just say they don't like having their secrets revealed. It's possible that this castle, along with the other, was erected by the Traveller on his way East, but nothing can be known for sure.