Interesting that the aliens had the high technology to come all the way here to build things— but didn’t know how to make a true arch.
Interesting comment. I've seen a reconstruction of what may be one of the earliest arches ever built, at Ephesus. Even my guide didn't understand the function of a keystone.
However, at a Hittite site in Central Turkey, I saw a tunnel through an earthen berm that had surrounded a Hittite city. The people had built the tunnel through the berm, possibly as a sally-port. It was obvious that they'd put up a wooden form, placed rocks on the form, then driven a triangular stone down as a keystone before pulling out the form, then building up the berm.
It's not just the aliens. The Greeks and Egyptians never had true arches, either.