Idea: Since the campaign AI is moddable...

[Parallel Pain]

Use your brains, it's a strategy game.

And yes woad&fang, just while that explains why they didn't guard it, it doesn't explain why they don't send troops to meet me once I'm on the other side (and on their rear too).

Besides, while having defended crossings, I've never seen the AI smart enough to permanently guard a strategic crossing with a sizable force, and certainly when they do accidentally put a stack on it, just moving beside the stack and ending turn will make it either move off or attack you. And there are plenty of places to bypass crossings.

And heck it's nice if you have to assault once in a while for a change.