The West is indeed over seas.
Getting ships to land is easier with the low technology level of Harkovast, since there isn't really anyway to detect approaching vessels except to spot them visually.
Also, just as the armies that comes from the West are vast enough to threaten all of Vellastrom, their naval forces are almost hugely numerous.
They are made doubly dangerous when commanded by The Mariner, The Evil of Water, possibly the greatest navel commander Harkovast has ever seen.