It really depends on what the circumstances of your war are. If this is just one city in a long line of cities you're about to take, just train up a couple peasant units. But some cities have to become your base of operations while you regroup and move on to another city. In that case, I reccomend archers, any light, fast skirmishers to sally out (missile cavalry and slingers are good for this) and lots of spearmen (phalanx if possible) to hold the town square.

You have to strike a balance between security and pragmatism. You don't want to waste perfectly good (and very expensive) armies by letting them rot inside city walls. But in some cases, leaving a peasant garrison is just inviting enemies to take your cities back.

so some of my cities that arent "on my frontier" just need a peasent garrison and the ones "on my frontier" should have a stroger garrison?

-thanks for replying by the way

And to counter some armys that have passed your frontier settlements (with ships or just walked), if you have enough money, hold per 3 or 4 settlements (depends on the size of the provinces) a small army, just some archers, spearmen and cavalry, to counter invadind forces and rebels, so reinforcements doesn't have to come from all over the map.
(so, if you control whole Italy and have highways, have a small army for the rebels and ,as example, to counter carthagian forces invading Italy.)

thanks I can really understand the importance of having a small force to protect a number of cities

-thanks

P.S. thats also pretty cost efficent

Ideally, if you're always forming or sending armies to the front at every moment, you should be able to slap together a defensive force if anybody sails over. But yes, good roads will make this much easier.

My primary occupation units are peasants. I will use a few cav units scattered about, and maybe odd infantry to assist against uprisings, and get me my new generals. Usually, creation and transportation, coupled with a general that can raise a merc army, are more than ample to put down uprisings.

If I have turn reason to believe a town must defend, I will give it better units for the short term. WHen the issue is settled, back to peasants.

What I do is keep peasants in my interior settlements and semi-good forces on the frontiers. I also keep a good army at each area, to combat rebels and incursions.