In MP you can win solely by amassing catas. For example i managed to win with catas ONLY vs agressive opponent who had longbows, I just had better production and massed catas. When he decided to attack in small stacks (longbow, spear, axeman) - it died anyway vs cata.
There is a deadlosck- catas are expendable at sieges, and they are veeery cheap. Make it expensive and you can't stop SOD, leave it like now and here we go - small superweapon.
The only reasonable solution here to me is limitation per square - like you can't fit 1kk army in 1 km^2 AND make catas more expensive. That'll solve at least some problems with SOD and catas.
Now Drill promo and colateral absorbtion looks pale - you need something like drill 4 to endure collateral normally and that render that unit significantly weaker - drill isn't that good compared to other - wich will take whole lot od damage..well, at least you'll move stack with 6 overdrilled units, that, in theory will absorb a lot of coldam...but to do so you'll have to BUILD that whole lot of units AND promote them.