https://en.wikipedia.org/wiki/Katskhi_pillar
Is that plausible, for medieval engineering? Are there other examples like this?
Nails. This single Roman fort in Scotland had nearly a million iron nails, all of them hand made:
> When it was excavated in the 1950s by Richmond, a large pit was found in the summer of 1960 containing 875,400 complete iron nails (Square shaft) ranging from 50–410 mm (2–16 in) plus another 28 (round shaft nails) weighing 7 tons, together with other iron objects, including cartwheel rims weighing a combined total weight of ten tonnes (Inchtuthil Nails by “Roddy Fraser”).
https://en.wikipedia.org/wiki/Inchtuthil
That's just one fort in a remote frontier of the empire. They were mass producing stuff like this, by hand.
Even assuming the chain didn't float in the air and instead draped down the rock and along the ground, it would likely take "only" 3-5 chainmail hauberks' worth of rings to cover the distance.
It wouldn't be an easy task, but there were far more than 5 people in medieval times with full sets of chainmail armor, so it's definitely not outside the realm of possibility.
It would take a lot more metal, though!
I don't know enough about materials science, but a 1/4" welded steel chain has a working load of 3150 lbs and weighs 0.63 lbs per foot. 1.5km is roughly 4900 feet, a chain that long would weigh 3100 lbs. So it would hold until the first wind or rain -- keeping in mind that these specs are for a modern forge-produced chain.