Artillery is going to be largely useless in this battle. The fort is sheltered against heavy bombardment, including aerial. The combat engineers(CE) will be the key to breaking through the walls. We can still shell them, but it won't have an immense affect.
Initial orders for CE: Two of the squads will swing wide and move up alongside the eastern hill/mountains; hopefully avoiding enemy fire. They will take one(more is acceptable) other squad of (unclaimed, unless someone volunteers) infantry as support; maybe a vehicle if available. When they reach a place near the wall but still not in direct firing lines, one CE squad will run to the wall/gate and plant the explosives. The infantry squad (maybe vehicle) will provide covering fire. If the first squad is killed and unable to plant explosives; the secondary squad will attempt to do the same. Otherwise, it will remain behind cover unless they are needed to replace the infantry as supporting fire. The final squad will move up with the rest of the infantry; attempting to destroy the wall/gate upon reaching it. (Final engineer squad is also available if someone else has a purpose they need engineers for. Orders revert to standard infantry commands once the wall is breached. (These commands are subject to revision)) EDIT: I can't read. -_- Okay. We get engineers after the mission.
(Something to note: If we're going to shell the fortress; the best place to do that would be at either of the gates. Those will be the weakest sections of the wall. And the most heavily guarded.)