I just read in the fortress log that a bunch of no-account bandits destroyed the western barbacan.
People with leather armour, swords and bows destroyed a many foot thick stone wall and giant iron gate???

No; That's just insane!
They need to fix this shit.

There is NO reason we should lose a building(+etc), short of an army with siege equipment... or equivalent monsters, such as ogres, attacking the stronghold!

I'm pissed off from discovering this!
Arn't you guy?

A builder with baggy shorts and no shirt could pull down a stone wall if it's not defended. 'nuff said.

Chisels and hammers man.

It is defended.
You can't even be attacked unless you have a baracks!

Having a barracks is not enough. You need hirelings with positive security stats. The higher your stronghold's security rating, the more chance you have of repelling attacks or averting things like theft.

I've been thinking about this for an hour and I've come to the conclusion that stone fortresses *can* be brought down quite easily. So easily, in fact, that I'm surprised that this culture bothers to build them. Please remember that this is a medieval setting that, incongruously, has gunpowder. (Not to mention magic of all sorts.)

You seem to be missing the point entirely. Doesn't matter if they have gunpowder.
A five foot thick wall built of solid stone blocks is both very very tough and extremely heavy.

If they had cannons, or other things designed to break down walls sure.
That is NOT what's happening.
A bunch of bandits ran up and destroyed it during auto-resolved conflict.
This isn't possible by any stretch of the imagination.

Recall this, that required no cannons at all (just 20 or so men working for a couple of weeks).

https://en.wikipedia.org/wiki/Battle_of_the_Crater

While interesting, not a valid argument.
They don't have explosives and they are being shot at by defenders.

I think the devs failed to account for any semblance of the walls actual durability and just did some random roll for weather something is destroyed.
If your mainline fighter (Eder) is around 700 total damage soakin, then a single block of granite the same size is about 7k of damage soakin.

Have a great life, sir.

Not at all, on the contrary it is you that is missing the point. Your stronghold does not have enough defense, so (in true computer simulated fashion), along come some baddies and knock down a wall -- this is not real life. Sheesh!

Wrong.
Everything has to make sense both to the game world and a degree of realism that's believable given the fantasy setting.
It doesn't.

So if you just don't build the barracks, there will never be an attack on Caed Nua?!? If I'd known that... :D

Because a talking throne that rebuilds your fortress without hiring construction workers is totally realistic.

You're an idiot looking for faults, plain and simple.