I would like the idea of 'projectile' units being enhanced in the following way:
Imagine, you own or have conquered a town, in which now the invading army and some of your artillery units are garrisoned (artillery = bowmen, catapults, .... artillery, and so on).
Now, if the enemy attacks, just one of all those 'projectile' units fires upon it. Very often, it misses. So, the defensive bombardment is of little benefit.
My idea would be, to have all 'projectile' units fire at the attacker, until it reaches 1 HP. Additionally, the attacker after that would have lost all mp's (to hinder it just from running away again).
Of course, for the next attacker there would be no more or less 'projectile' units to fire upon, but this would simulate the attack waves...
The benefit of my proposal would be found in the following case:
You just took a town with two of your attacking units left. Since your artillery would be left undefended in the open field, you move it into that town again (let's say, five artilleries). At the next turn, the enemy counter-attacks with an army of three fast-moving units. One of your artilleries fires upon it, then your gunners go to lunch, while the enemy army chops down your two 'normal' units, taking the town back and getting the five artilleries as a bonus.
If all five would have had fired, there would be a good chance that the army wouldn't have succeeded, thus you would have hold that town, and all units would have participated at the fight, as it would be expected.
Furthermore, since the enemy army was subject to heavy bombardment, it's attack broke down and it's rests have dug into the ground, just in front of your troops.
At the next turn it is your decision whether you will give them the final blow (could result in weakening the city defenses) or if you spare your troops for further defense (then, of course, the army could retreat and recover in their own territory).
If the enemy would have attacked with one cavalry, that one would have been redlined as well. Maybe even a second cavalry would have suffered the same fate, but for the third army there would have been no artillery left.
Anyway, it would have become very costly to attack artillery-protected areas.