The definite way to get rid of SoD's, as used in history, is:
A) Shell them with artillery
B) Bomb or strafe them using aircraft
Is true. Unfortunately, this is not accomplished in the game thanks to the cap of 5 units being affected by collateral damage. So, I agree with this:
CIV 5 should remove the limit for collateral damage from artillery (just like in real life).
The more units are in one area the more units will suffer collateral damage.
Here's my idea for a specific way to implement this:
Siege (and air) targets a randomized number of units in a stack, between 25% and 75%. After the targeted units have been selected, the units with immunity to siege collateral are removed from the calculation; also, units with drill II have a 20% chance of being removed from the calculation, drill III with 40% chance, drill IV with 60% chance, etc.
For the remaining targeted units, damage gets dealt to them according to the following formula:
R * (S%-((100-X)%)
Where R is the amount of damage the targeted unit would get from losing one round to the siege unit in ordinary combat, where S% is the targeted unit's current percent of its total strength (a knight at 8.0/10 would have 80% of its strength), and where X is the siege weapon's damage cap.
Let's say we are using bombers which have a damage cap of 50%. At 100% strength, a unit would get R * (100% - (100-50)%), or 50% or a round of combat in damage. A unit with 75% strength, being besieged by a bomber, would get 25% of a round of combat in damage. A unit at 60% strength would get 10% of a round of combat in damage. As units are bombed by bombers, they would asymptotically approach (but never quite arrive at) 50% damage. Towards the end (when a unit is around 70% or 60% strength) you will start getting really bad diminishing returns from bombing them further. For using a siege unit like artillery, with an 85% damage cap, you will be able to do more damage to the units before the diminishing returns really start getting noticeable.
The next part applies only to land-based siege:
To determine if the siege unit is damaged, the best defender of the defender's stack (who would normally be facing off against the siege unit) is hypothetically paired up against the corresponding best defender of the offensive stack in order to see if the best defender of the defensive stack can execute a successful sortie against the siege unit. A fake round of combat between the two best defenders is "fought" (with no damage being registered on either side). If the defender of the offensive stack wins the round, then the siege weapon gets away without any damage. If the defender of the offensive stack loses the round, the sortie goes forth and the siege unit is then paired up in normal combat with the best defender of the defensive stack, and they fight it out as normal (although with the sortie-ing defender getting a 50% bonus because the siege unit had been busy with its besieging and is getting caught by surprise by the best defender's successful sortie attempt) until either the defender reaches its damage cap and the siege unit retreats, or until the siege unit is destroyed.
So, for example:
A stack of 8 crossbowmen and 1 catapult go up against 4 longbowmen in an open field. All units are without promotions or defensive bonuses and at full health.
The catapult attacks first. It randomly selects between 25% and 75%. It selects 60%. That's how much of the stack it will target for collateral damage. This is rounded down to 50%. 2 longbowmen are targeted.
The relative strengths between it and all of the longbowmen are 5 vs. 6. So, however much damage would be done per round to the 2 longbowmen, that much is dealt * (100% - (100-75)%), so (1 round * 75%) damage is dealt to the 2 longbowmen.
Then the best defender of the defensive stack (one of the undamaged longbowmen) goes up against one of the crossbowmen for a fake round of combat to determine whether the longbowmen can sortie against the catapult. The longbowman and crossbowman are at even odds. The longbowman happens to win the fake round. It gets to sortie vs. the catapult.
The longbowmen fights against the catapult with relative strengths of 6*50% + 1 first strike vs. 5, or 9 + 1 first strike vs. 5. The longbowman takes some damage, but annihilates the catapult.
What all of this means is that:
*Dividing stacks
*Stack defense
become much more important. The SoD is no more!