For ancient/classical/medieval?

What kind of army composition for example? Tatics to take out cities, etc?

1/2 army is catapults, then troop mix. Swords and spears/axes (or elephants can replace spears/axes if you can get your hands on them).

Bombard defenses down, collateral damage with siege until you get good odds with normal units, attack, kill.

Swords and catapults get city raider. Elephants get combat. Spears would get combat until you can pick up anti-mounted promos. Axes, as stack defense, would get combat, though you can certainly use those on the offensive as well if you want to give them CR, albeit weakers vs non-melee than swords.

In middy times, maces take over for swords/axes, pikes for spears, and elephants are still useful! Knights are OK as supplements also, or if they're a UU for you. Also, mix in some trebs with your catapults, but half your army (roughly) should still be siege. Pults are better in the open, trebs are better vs cities.

Half siege is a good rule of thumb. The other half of your army will depend on what your opponent has for an army... If he's heavy on mounted, you want more spears/pikes/elephants than city attackers like axes/swords/maces. If he's heavy on melee or has no horses, then you want more axes/xbows/maces with only a couple anti-mounted troops "just in case." You also want to bring some archers/lbows, a couple with guerilla promotions, the rest with city garrison so you can leave them in cities after you capture them. The nice thing with war elephants is that if you get them, you really don't need much other than lots of elephants and lots of catapults, with a few axes/maces as insurance against his spears/pikes. Siege is gonna die en masse though, so make sure you're cranking those out back home. If you're doing it right with the bombarding and collateral damage to soften up cities, you'll rarely lose one of your normal troops.

Cats & Trebs

Whatever else in the stack is only there to protect the Siege weapons and to do the actual taking of the cities (damn you BtS for nerfing all-Siege SoD's).