Aren't the spies uber-powered during the sieges?

[Zenicetus]

If the spy opens gates, it's all the gates. This can be very useful for splitting up a big defending army by attacking through three gates at once, or running in a cav unit quickly through an undefended gate.

Personally, I don't think it's overpowered because 1) the enemy can do it to you too, and 2) it does take time to train up one or more spies to the point where you can rely on this, instead of bringing siege engines, and 3) I've hit many cities where the enemy had the place filled with their own spies, that thwarted the strategy when I arrived.

P.S. one additional thought on play balance... I think all these things like gate openings, assassinations, etc. are balanced for players who don't try-and-reload when a spy or assassin attack fails. For example, the first time I sent a pretty good, mid-ranked spy down to Marrakesh to both spy on the city and maybe open the gates, he died instantly when entering the city. I re-loaded and found out that there were four enemy spies sitting in the city. If I hadn't loaded a save game to restore my spy, it would have made that attempt much more expensive in terms of resources and campaign time. So whether it's balanced or not, does depend to an extent on how you play the game... i.e. whether you take your lumps with failed spy and assassin attempts, or just reload and try another strategy. BTW, I never did manage to get a spy in there... I had to take the city the hard way.