Step 1: Gain control of one of the five castles.
Step 2: Cut a deal with mayors and bribe them to impose a 100% tax rate on their nodes. You won’t need to worry about this if you are the mayor.
Step 3: Rob all possible caravans that go to other castles
I wonder about the dynamics of the populace under such a king. What will they do?
Will the common people move elsewhere?
Will they become bandits?
If you’re enemies with the guilds under the castle’s zone of influence, it makes sense to follow this plan!
Declare war on every possible guild in the richest node, completely disrupt their farming, force their mayor to set the taxes at 100%, and start draining their resources.

I don't remember every little detail of every system, hit me up to fix any issues in the plan.