They would still have to wait a few seasons before launching in order to get a good many soldiers and enough supplies to last the trip considering things, plus they have to build a fleet. The raiders like raiding, it stands to reason, so a few ships might mutiny considering the boring wait.Leaving two maybe three ships around, considering each one may hold a regiment's worth, that's only three regiments, which we equal+a ship+walls+whatever we muster.
Plus would you really let a fleet of barbarians rest in your harbor? I mean really? Would you let a plunder-happy band of men into your town willingly? Its like invinting rabid wolves into your house so you can kill the neighbor.
Paying them as mercs is alright and all but would you serve with them, they would destroy the infrastructure of the town your trying to conquer.
The Jarldom has no docks,whereas we do, so they will have to pay more money to get some built or upgraded. Plus a large force of transport vessels are easy prey for pirates,which considering there is a one out six chance of meeting, you would be tempting fate.
Plus the Jarldom's nautral soliders are raider units, whcih are slightly weaker then regular infantry.
If the Jarldom attacks this city, the occupants will have enough time to fortify the walls, muster a few militias and hire some mercs before it arrives, and then, like I said if I get what I want, their fleet will arrive crippled and weakened. They are in checkmate. They can't do nothing to us, we can't do nothing to them.