Unit Movement problem.

Well I'm making a Castle defense map and I'm trying to find a way to make the computerized enemy units move to and assult the keep.

Therin lies my problem. The only accessable routes to the keep is through gates. I Know how to get them to attack-move to a point but I do believe they will ignore attacking the gates to get to that point. Is there a way to get them to move to the attack-move to a point and destroy gates along the way?

Use waypoints at the gates. Units reaching a waypoint attack the gate if it is still standing, and continue to the next waypoint if it is not standing.