Quote Originally Posted by Musashi
I think they likely help... that doesn't mean you're immune though.
With a high level spy, or stacks of spies, I think you're immune. I tried to send a spy and an assassin with 9 ranks in subterfuge (a real death machine) into Marrakesh, and they were both discovered and killed. After reloading, my spy parked outside the city could see that there were four spies inside the city, fairly low-ranked I think, but I couldn't be sure. My assassin couldn't touch anyone in the city, couldn't even do sabotage.

What's interesting is that he couldn't touch anyone standing outside the city, either! There was a low-ranked Moor general standing next to the city, and my assassin showed an 84% success estimate. But he failed every time, discovered by those pesky spies in the city. So there's a radius effect for spy discovery, not just the square they're in. I assume that applies to army stacks also. After the Moor general moved further away from Marrakesh in a later turn, he was an easy kill.