I’ll assume you mean NPCs, not thralls. The answer to “is this really that hard to do” is quite often “you would be surprised”
Personally, I really dislike this idea. There’s a reason why fortifications have walls and why attackers historically needed siege engines and ladders to assault those walls. While I acknowledge that climbing is a useful mechanic to give players, it’s highly unrealistic and having NPCs being able to climb would both break the immersion and ruin PVE balance.
If I could have things my way – and it’s probably a good thing I can’t – I would restrict player climbing so that it’s only possible with the climbing gear. But that’s just my personal prefernce.
I already suggested this once. It was wildly unpopular