Lol, I can't believe ppl didn't get it yet.

As EaglePhoenix said, just walk to the portal, and STAY in front of it (in city, yet), now, check the horizon, u won't see anything on there, just move your camera or wait sometime, the castle will pop up in the horizon, behind de portal, now, u can pass throught it and port to the king, not to the river.

It happens because there's a small bug, idk why, but the castle must be downloaded, the 100% gets when the castle pops up, after it, u can travel to other cities and stay logged for any period, the castle will be there.

Idk if this applies to other similar bugs, cos idk if there r others lol