Need help with AI and pathfinding problem.

I have a problem with my current map of Helms Deep for Middle Earth Total War.

I have finally got a playable, real, accurate to Tolkien Helms Deep set almost finished. My last hurdle is the AI and pathfinding which is a right pain in the rear.

Basically my problem centers around collision and pathfinding outside of the city borders. This isn't something which is generally touched on in the game as all cities are on flat boring land. With Helms Deep I have the long causeway leading up to the gate and so have put collision data and pathfinding around it to ferry the troops to the gate. The main problem is that the pathfinding doesn't seem to work on the troops outside the city borders and the ram seems to ignore collision data so gets stuck on the hornrock where it shouldn't be able to go.

Does anyone know how to make pathfinding data outside of city borders?

Does anyone know how to make collision data that rams will be affected by?