This is a very valuable remark.
The AI is not able to understand bombardment. It will send in hordes of units only to die at your defensive strongpoints, if you use your early "artillery" well enough.
Very true. A stack of cats or trebs can keep the AI away.
The same stands true when your're on the attack. A stack of 10 cats, 10 spears and 10 bows (just as an example, whatever may be the needed combination) will get you almost any city in the early game.
That's 600 shields of units. Does anyone ever really build 10 catapults in the ancient era? I'm usually busy building workers, setters and warriors for MP. And with all the mad trading that goes on in the AA, the AI and I are typically out of it before long. If you have built that many units, that AI will have a similarly nasty SOD up its sleeve to go after you. You need a reserve of artillery and attackers for defense. Given the choice, I'll take 20 horsemen. Better yet, I'll take 10 horsemen and attack sooner before the AI can build up a big stack and get to pikes
I find the best time for the big slow stack is the early medieval era when you are no longer losing population for workers and settlers and your cities can now grow to be able to crank out those slow movers.