The divisor for a wall spell is 8. This reflects the structural integrity of the wall. Multiply the modified force by 50 to determine how much normal damage the wall can take before suffering significant loss to its structural integrity. When its points are all gone, it's a pile of rubble.
However, walls have armour, as well. Take the undivided force of the spell, or multiply the modified force of the spell by 8, whichever is less. Divide the result amongst Coverage, Wound Conversion and Damage Reduction. So, Rook's Castle Wall (24) has Cov 8, WC 8, DR 8, and can take 150 points of normal damage before losing structural integrity.
Uriah's Wall of Bone (64), however, is not as good at coverage as the castle wall, on the other hand, it's one hell of a lot tougher. The Cov is 13, WC 26, DR 25. It can take 400 points of normal damage before being breached.
Siege damage is different to other forms of damage, in that it will put one hell of a hole into a structure. Normal damage can, at best, and assuming a co-ordinated team, really only create a man-sized hole in a structure. Siege damage is quite a bit more exciting than that.