Same as with 1/3 rule
Keep the 1/3 rule, but cap it so that you can attack anything with ~1500 troops.
It's totally bullshit to be forced to attack a castle with 3000 troops and lose them all, while only 1800 got killed.
And maybe increase the catapult damage by max 25% if you're right next to the wall. And reduce it ~25 - 50% when you're so far that archers can't hit the catapult reliably, I know the "archer range" is not very accurate, but you get the point. + outside the archer range it's really easy to build multiple catapults and they don't really need anyone guarding them besides the ones actually operating them.
And about siege towers. It takes quite a long time to break them with melee weapons, so I think HP-wise they are fine against melee. But I do agree that the siege tower shouldn't break from the fist 2 hits from a catapult, not because of realism but because of balance. Building a siege tower takes much longer than building a catapult = when the siege tower is built, the defenders probably (if they are smart) have their catapult already done as well + it should be aimed for the the siege tower too.
Also building a siege tower means that almost everyone on the attacking team is carrying const.mat while defenders only need 2 people to build the catapult.
And ladders are fine as they are IMO.