Ballista Towers and ballistas are not exactly choice iron age units when playing someone who uses cats. Defense is great, and in iron age even tho it wont stop siege weapons it will stop HAs if he decides to tranny over and raid your town.
If you get a really quick iron when walled in (when I say quick, I am talking sub 23) use horse archers. If the civ you are using doesnt have HAs, then this is pointless. Take the HAs and run them into his town and only kill villagers. Repeat the process. HAs are some great units.
Back to the original topic. Try building cavalry, and if he counters it with camels build archers and retreat into your towers. If you manage to get ballista towers extremely quick (I question the worth of getting this upgrade as the first super unit upgrade) then plop those babies down in the battlefield. This prevents him from building stuff near you, and will chew up every non-siege unit.
I would suggest playing choson if you are this type of player. Even tho they have a weak bronze, if you can use those extra range ballista towers along with choson long swordsmen/legions, and ballistas, you have yourself a choice army. I would even try throwing in a few choson priests, and hiding them behind those plus range ballista towers. Try to lure their guys into your towers, and seperate their offense in this fashion. If you manage to take away their horse archers, for example, and lure them into the ballista towers they will get chewed up. But now you can lead in your swordsmen/legions and get all the foresaken catapults.
Basically use a unit combo. Defense is a very important factor, and if you want to go for a fast iron then defense is essential. And dont wait too long, as soon as you get guard towers start laying those all over the place. I would save supplies for the upgrade to legion, personally, but its up to you. The ballistas would probably be used to mop up and cavalry but DO NOT use ballistas to take out the catapults unless the cats are completely distracted with the towers.
Whew, hope that helps