Well the reason I mentioned this is because, there is a big difference between a fort and a settlement.

I understand why spies should be prohibited from taking settlements, no question there. Spies opening up gates and avoiding the seiging process will morph the game into a blitzkrieg, depending on how many spies one person has.

Having said that, Forts, on the other hand.. I'm not so sure we should be quick to rule spies out in capturing these forts.

Forts can be build anywhere, especially strategic locations, chasms, bridges, rivers. and instead of a bliztkreig, we end up with a drudging-sluggish war of breaking through forts to reach your desired goal
Those who build the most forts, at the best locations, will have the advantage over others.

However, if spies are allowed to open the gates of forts, which they won't always be successful and can be defended with other spies, it would allow spies to be useful, but not overpowered since prohibition will still be in effect in regards to settlements. So while you will still need to besiege cities, and settlements (which are very important) forts that are easily built and put in many diffrent locations, won't guarantee 100% protection,

It's just a thought, Thats why I feel that spies would be the great equalizer