And then again about the Protective trait..
Yes, I think walls and castles are weaker than barracks and drydocks. Even playing defensively, barracks are higher priority than walls. Castles I build rarely if at all (if I go for engineering early, I might build walls and castles to some cities that would gain large benefit of the extra trade), drydocks are similarly fairly rare (I build them to productive coastal cities that I might want to build some ships in). Aggressive wins on the buildings.
On free unit promotions though, I don't think aggressive is that much better than protective. The reason aggressive players devalue protective promotions is probably that they don't value archery troops but do value melee troops.
Archers are mainly used against barbs anyway, and axes do the job just fine. Axes also benefit from the barb-duty xp in that they're going to be used in offense. Archers aren't. However, archers can go for drill line with the barb xp, get promoted to crossbows, and become offensive troops that way.
Longbows I feel are good only for city defense. And honestly, I don't expect anyone to get so far as to actually attack my cities - except newly conquered ones.
Crossbows going the drill line can get to Drill4 with 10 xp if they start as protective. And at that point, they're a real force. Of course you still need siege machines, and you still need spears/pikes, but you don't really need that many CR swords/maces.
I don't go for combat line in promotions. My city attack troops go for CR, my longbows go for CG. Combat line is needed to unlock counter proms (some shock axes/maces is good, formation spears/pike/'phants is good). Drill unlocks these just the same, and crossbows are better anti-melee troops than axes anyway, so I definitelly will value Drill1 Shock crossbow higher than C1 Shock axe. What's left is the anti-mounted troops, where if possible I'll opt for formation 'phants - and neither aggressive nor protective gets promotions to mounted units. Yes, aggressive gets formation spears/pikes easier due to free C1. So they should go for those instead of 'phants I guess.
Maybe some beefing up could be done. But what exactly? 20-30% strength increase in one's cultural borders is simply too much. Higher fortify bonus might ok (but I'd just double it so it's reached in the same five turns). I don't think barb bonuses would be that good an idea. Healing rate bonus - maybe. Forts suck any way you put them, so bonuses to them don't sound useful.