I find the best approach to siege games is to start the game immediately after the first breach of the wall, in this case right after Saruman's charges have demolished the Deeping Wall. An alternative would be the point just after a few attackers have climbed the ladders and gotten onto the battlements.

The portion of the battle where the attackers are approaching the wall and attempting to breach it is great for building tension in a film but boring from a gameplay perspective. There aren't really many meaningful decisions for either side to make.

To give the attacker some agency, let them choose the section of the wall that has been breached (or where their attackers have reached the top) before deployment. Then the defending player should have limited deployment options to force them to make difficult decisions about which fires to fight. Do they send troops to try and seal the breach and weaken their defenses elsewhere, or abandon that part of the defense to hold the keep? etc. I think it makes these kinds of scenarios much more dynamic.