Tip - how to capture cities/castles w/o war

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.

Was already the case as far back as MTW and Shogun. Doesn't work so well once the enemy builds spies of his own though - or Border Watch in MTW.

At least without patch I saw just 1 (one) city with a spy inside for a week play. And this spy left after 2 turns.

Yea, I was playing as Sicily and the HRE was starting to pull off the same trick against me at Naples... I had to train some spies to counter the HRE and keep Naples from going rebel

I can usually get cities easily from diplomacy when the AI declares war on me then realizes its mistake and asks for a ceasefire. And they usually have really strange units in the cities, like 10 catapult units and nothing else.