that rule applies already, if you choose to shoot cannon at the walls then you can but what about line of sight? and then the siege equipement? you need to protect them against attacks and keep them safe until they get to the wall
attempting to move the siege equipment costed 2 turns and in the end I got the battering ram and the movements blocked the LOS of the cannons for 3 turns total, 4 turns to knock out the 2 warriors supported by 2 sorcerors and a hero on a dragon.
I have to admit, it was a lot of failed orders by the dwarf heroes that made it so disastrous for the player