How can I prevent spies from opening my city gates?

[Komutan]

I am playing as England on VH/VH settings. Caen is almost always under siege by French, Scots, Danes and Spanish. I have repelled maybe 10-15 assaults. The problem is that on all these assaults the battle starts with "We have been betrayed..." message and I am not given opportunity to deploy my troops.

I know this happens when a spy opens the gates, but it happens on all assaults and no matter which of the factions is attacking. I put 2 spies with high rating into Caen as counter intelligence and I sometimes catch a spy but still all the sieges start the same way.

Is there anything I can do to prevent this? Or maybe is it some kind of bug? Spies opening the gate on all assaults does not seem very likely.

[Whacker]

You might have an infestation of 'super spies'. Stick a few of your own spies in the city to counter them and increase the chances of them getting caught/expelled, and then use assassins to take care of the ones that live. Optionally, if you don't have assassins, use the cheapo 'box-of-8' surround and kill trick.

[WhiskeyGhost]

it really helps if you get some of the public security traits/ancillaries and also, make sure your spies aren't junk ones, 1-2 skill spies wont have much luck stopping spies with 10 skill

[Agent Smith]

I am I weird for actually LIKING when spies open my gates?

I find defending the gatehouse to be preferably to any other defensive position. In my battles, two spearmen in schiltrom stuffed in the gate entrance can defeat by themselves almost an ENTIRE stack of troops, especially cavalry, stuffing through the gatehouse. They may need support from one other unit, but usually they doe fined.

Usually at least one spear unit deploys by the gate in a standard set up, and a second spear unit can usually get there before the enemy does. The enemey AI always drops all of their siege equipment and just rushes the gates. I usually don't mind that at all