There are a few triggers for seiges that are known. If a seige is successful, the Prince, or mercenary will try his luck again rapidly. (The book A Distant Mirror is a fascinating study of the times, with bands of uncontrolled mercenaries demanding bribes and doing freelance seiges across Europe). Selling him goods at the gate of course softens his anger, but doesn't affect the marauders. When the prince asks for a loan of goods, he usually needs them for his army and a seige somewhere will follow. Don't loan, obviously.
Now for the speculative items: Assuming needs like beer in sufficient quantities are provided for and the soldiers are kept happy (selling iron goods is necessary too, look closely at the other equipment the soldiers carry. In a town with no or few iron goods they have little, sell a bunch of iron goods and you will notice in a few days they are carrying other items now.), have the players smuggled more advanced weapons to the weapons dealers across the Hanse in an effort to prevent seiges? Since the shipyard and the weaponsmith are linked, a force of bowmen are less likely to fend off an assault than a cadre of musketeers, and appropriate sales of crossbows and muskets may be very useful. I presume that the players have joined the respective guilds and made motions to increase the numbers of soldiers to satisfactory levels as well? A 10/10 or as 20/20 won't do as well as the 40/40/40/40 full defense allotment. Even the 20/20/20/20 with full wall defenses wins half the time or better.
I do wonder about a few other things as well; are the "honest" traders experiencing the same severity of seiges? A cog in the harbor serving as outrigger isn't the same as a cog and two or three cannon towers. Perhaps the game somehow in its programming lumps all buildups like pitch throwers, cannon gate towers in with the harbor defenses and count for the city as well defended/poorly defended. And similarly, a neglected defense buildup of the walls and the towers with a rich treasury are an invitation to plunder, like an unarmed cog full of wine and iron goods on the sea with a crew of twelve on board? It would seem so. I would like to hear, if there are any honest players around that is!
And regarding trade with the prince, a healthy competition in the hometown may be trading goods with him (they don't load all they produce on their ships, nor does it all end up in the marketplace either), while an impoverished competitor with no ships and no businesses either may be neglecting the landlord and be a further trigger to seiges. Maybe, just maybe, letting him have a life, and the ability to bribe the invader if he is mayor, just might make your life easier too. Pure speculation of course.
Lastly, making motions to lower the tax rates and thereby present a less inviting target may be appropriate in seige prevention. And in a flurry to build, when we don't allow the town enough time to build its gate towers, we may be sowing the seeds of our own destruction.
Comments?