The Catapult's primary purpose is to weaken the defense of Defenders, so your other units stand a more fair chance when moving in to take the Cities. It's possible you'll lose some in the initial attack,but you can't expect to get in without any casualties.
Master_Pantz posted... what do you mean by stack them?
maybe i'm wrong but i thought i had it on the same square as my other armies when i used it to attack and only it and the enemy was involved in the battle --- http://card.mygamercard.net/gbar/live/Master+Pantz.gif Most Wanted: Operation Darkness, Halo Wars, Too Human.
I'm in the same boat. I had 2 catapults stacked with 3 legion armies and the catapults got smacked down before even getting a shot off! It would be nice to be able to combine it with the armies into a combined arms unit.
You need for form an army of 3 catapults. With that catapult army (protected by an archer army), you can attack other cities much more handily. Improve your odds by sending in a spy first to remove defensive bonuses. =)
Other units only protect them when you're getting attacked yourself; I don't know the exact mechanics, but when being attacked it seems that it always puts your most defensive / healthiest units as the first targets for enemy attacks, so Catapults should be dead last if an enemy is trying to attack a particular square.
When attacking with Catapults, there's no inherent defence from other nearby units, so you want to make sure you have the initial attack weighed heavily in your favour, through the use of terrain, Great Leader bonuses, or the use of Spies to weaken the defenders.