In response to the first part: Artillery just hampers your troops, requiring some slow defenders and the artillery to come along in a big stack, and as soon as the enemy sees it, he or she will have ample time to bring a defending army of rapid cavalry to hammer away at the stack, whereas if you had only used cavalry, their reinforcements might arrive too late to stop the cavalry.
I assume you use the term "army" loosely here, to mean just "a hoard of units." The AI seldom uses true armies. And if the AI is dumb enough to attack this stack, your mission is accomplished. You get to bombard him to oblivion, leaving your own cavalry unmolested to mop up the redlines and take towns. After all, you are seeking to defeat his military. What better way than to lure him out onto open ground and in range of the guns? If he attacks with his fast units, so much the better. Now we don't have to chase them down.
What's more, if you're using cannons, (in shields) 1 cav=2 cannon, 1 cav=1 rifle= 1 1/3 musket, so making a stack of artillery and defenders (unless you draft
) is less efficient than a massive stack of doom of cavalry, that will kill riflemen so easily by sheer numbers.
I disagree that it is less efficient. First, we'll build more guns than we do leg units (at least 2:1 in our stack), so we'll make maximum use of the 2:1 production efficiency and will produce roughly a third more literal units with same shield output. Second, the artillery can be built in our perpheral cities where we can't afford to build barracks. Essentially, the production used to build the artillery is production we don't have an immediate use for, anyway. Third, and by far the most important, WE DON'T EXPECT TO LOSE THESE UNITS.
This is what I meant in my response to your point about longbowmen, and something you are also missing about the cavalry. It doesn't matter what the specific units are. When you attack with cavalry, or longbowmen, or any normal attack unit against a prepared position without artillery bombardment, you WILL lose some of your attackers. Unless you have a significant tech advantage, you are likely to lose quite a few units, since the defender gets an advantage in combat. Each unit you lose needs to be rebuilt in order to keep the momentum going. You must count that as part of your cost of war. With a stack of artillery hitting them first, you won't completely elminate the losses, but close. Your stack of units may need to rest and repair from time to time, but it will never (or at least very, very seldom) get any smaller. All that production you are saving can be used on city improvements, wonders, or more units to make your military even bigger.
This saved production capacity should not be underestimated. If you use artillery judiciously, you can actually find yourself considerably stronger at the end of the war then when you began it, and moving right into the next attack. Far from stopping your conquest so you can rebuild units, you will be tearing your hair out because your artillery stack is too slow to begin hammering your next victim right away. There will be a constant mental pressure to just send the cavalry on ahead anyway. At which point you will lose some of them, and find yourself wasting valuable production and time by having to rebuild again. If you spray cavalry willy-nilly everywhere, you are doing yourself a disservice. There are times when it is best to let the cavs loose and rampage to the enemy rear. But there are also times when it is best to put the reigns to the horses and let risky opportunities pass you by in favor of the no-loss sure thing.
Artillery requires a peculiar kind of patience to be used effectively.
Edit: I see I cross-posted with Aabraxan...