Well armies are not as strong in vanilla as they are in C3C, but they still are useful. Muskets cannot attack, for all practical purposes. To attack knights and pikes, you will need to ping them to at least yellow and use knights. C3 does not have MDI as I recall.
If you do not have knights and are stuck with muskets to attack, then pair them with lots of cats or whatever you have. I like to use stacks that cannot be attacked and cannot fail to win.
In C3C that means armies, but without them you need bombardment and lots of it. If the price of Longbows is less than muskets and I am sure it is, use Longbows to attack, if no knights.
Hopefully you can get a leader to make an army. I know in C3 it is hard to make armies as leaders can rush wonders, but a few armies are worth more than a wonder in a war.
As you probably know, the AI will not attack a knight army that is healthy, even with cavs. This means you can move stacks covered by armies and never risk attack. Just do not get next to a town with cats, until you attack.