Let's take the case of Stonehenge

Radiocarbon dating indicated that the monument was erected or built between (3100-1600) BC.

Were the barbarians  or natives (not the Celts) then capable of such a monumental task?



Or were they aliens (if not ET what other civilization)  footprints left behind?

We do not know what sort of technique they used during that time because the methods are lost to us. However, this does not mean that humans are incapable of achievements. Moreover, we still do not know the extend of their knowledge in architecture. Also, there were numerous attempts in building pyramids showing that they were trying new ideas from time to time(for example the Bent pyramid and the red pyramid). I think it is a pity not giving the ancient Egyptians the credit they deserve for building such magnificent structures.

Because there is no need for one? Currently there are much simpler ways to construct large monuments. Why follow the hard way?

There are many pyramids around the world(in different countries)  and each of them are years apart. This shows that pyramid building is not unique to the Egyptians. Also, if the aliens did descent to show how pyramids were build, then these different pyramids from different part of the world would have been of around the same time but this is not the case when the carbon dating for all of the pyramids in the world are many years apart.

Pumapunku does not prove anything of what you said about aliens. Like I explained earlier, the technique is lost to us. It does not explicitly shows there is alien interference. At the moment, I can only see whatever you think that seems impossible to build is of alien tech. Didn't you consider that it is only the technology or the skills that were just lost to us?

It is possible but to support such notion it is better to have a concrete proof, rather than "I don't know, therefore aliens."

The usage of pyramids differs in different cultures. The Egyptians used it to bury their royalty while those in the south/central America may have been used for religious purposes (like sacrifices). It was speculated that the ancient cities of south/central America were abandoned  because of climate change where a drought happened, making crops didnt grow.