It is so easy - I feel like cheating:
As I said in a previous post - the spies are overpowered - they easily open the gates and you capture the city in the same turn...
BUT - SPIES ARE MUCH MOR POWERFULL - they greatly reduce the enemy happiness. Simply send in any city or castle 3 spies and demolish the "happiness" buildings with couple of assasins. Aften 3 turns at most the city becomes rebels for you to grab. Or you can send 5 spies w/o any assasins. Right now a band of my spies and assasins are cleaning the Earth from the spaniards. It is so easy I feel achamed to use it. It will take a whole 10 turns to finish the mighty Spain. Don't know what will happen when they remain with only 1 city - capital.