Ok, I agree that has to be somehow by boat, but I've rocks and other abandoned boats that prevent me to go in the direction of the 4th castle... Maybe there's to buy some artifact that helps you to pass the rocks? I'm stucked!
Oh well i tried for some time to get there,but since i was pretty much on a deadline [I was in a rush]I simply wrote add_all_spells cheat , used the Vessel of Shalasa spells and set sailed from the rear of the 3rd Orc base[there's an opening there].As for a method without cheating you probably have to wait a long time untill you get the spell or you get something else.Anyway use this way if you want and if you want to get there without cheats feel free to find a way of your own.
Thanks for the reply, I've got already the vessel of Shalassa spell, without cheating, and I'm trying to send it from the place you're telling me. I've also taken two occupied boats the closer that I can, to that point (the arbour, the roks) but the spell is unactive... I'm really ****ed!