As the title says, "melee" siege engines should have a movement speed proportional to the amount of units pushing them. So if 4 men are normally used to move a siege tower, killing 1 of them should slow it down by 25%. Of course we could go a more realistic route and say that you need at least X amount of men to be able to move the engine in the first place, but a proportional decrease is good enough for me. Currently only the death of all pushing units manage to stop the "melee" siege engines.