From the article: It involves a infinitely long and incredibly steep climb of seven hours
There must have been hostile tribes that they were defending against. So they built there fortifications high in the mountains.
So why did they abandon these secluded fortifications? They probably conquered their enemies in battle. They were then safe to settle lands at lower elevations without the highland fortifications.
It was a summer getaway for the big cheese; also, there *is* no hospitable geography in much of Peru. :’)
Well, obviously, they were concerned about artillery. They built in the high ups to ensure that no one could fire down upon them from above.
Simple, really.
Or, maybe, they took the “moral high ground” metaphor to absurd lengths?