Re: how to cripple your enemys economy
I am not very clever at the economic warfare. I simply besiege all of the cities that I can afford to move against. Do not assault, you do not even need sufficient strength to necessarily win. If a field army moves against you that you cannnot handle -- either fight and accept defeat, but hurt the enemy as much as possible --- or simply refuse to fight, the AI does not always follow up again on the same turn. This is better than trade disruption, though do that also. Why? AI cannot build troops where needed due to the seige and the AI's total income is greatly reduced.
If a city is lightly held, then assault. THe key is to take out the cities and leave the castles. If you desire to sell a settlement back in an effort to get a ceasefire -- convert to a castle first.