I think that this has been mentioned before, but I just wanted to mention it again - Spies are amazing weapons when attacking cities. When you successfully "Support a City Revolt", the city's defense percentage drops to zero for one turn. You then move in with your siege weapons to soften up the defenders and finish the city off with with your Maces, Axes, etc. all in one turn.
Sometimes it helps to bring more than one spy just in case one gets captured or fails to "Support a City Revolt"
Sometimes it helps to bring more than one spy just in case one gets captured or fails to "Support a City Revolt"