I have a fledgling concept in my head where some point around 100-300 BC, a outpost from beyond the moon is set up in the great Plains around the Mississippi River in what is now modern day St Louis.
However, I am looking to avoid stereotypical trappings so they're not greys or martians or nordics or lizard people. Or be responsible for any human megalithic structures.
The nature of the colony is very different from a human one however. Essentially the alien colony consists of a large tower-like structure that's kind of like a tether and ball, if said ball was a bulb with a core of anti-gravity material (making it sort of akin to a space elevator on a much smaller scale, since their colony's tower only stretches 10 miles into the sky). The details of the engineering I couldn't even begin to describe so please don't press me on the mechanics. I don't know, it's alien technology centuries if not millenniums more advanced than our own... It's like asking someone from Papua New Guinea without any western/modern education how an iPhone works.
The layout I can describe however. The bulb itself is where the inhabitants live, they easily are able to fly out of the bulb and are able to travel around the world no problem in theory. In practice they deliberately have isolated themselves from the Old World where human activity is much heavier. They have no problem affecting the cultures of the New World, for they view new World humans as more or less blank slates compared to the somewhat deeply rooted human civilizations such as Romans, Mauryans or Chinese that exist in the Old World. Or really, they see the new World as less populated by native sapients and easier to manage.
This outpost is from the Vamusk species (this detail being important to me and me alone), who are essentially arboreal bipeds (imagine if you will, New World monkeys that made a space faring civilization) that aren't really individual minded. They are synergistic in thought instead, "connecting" minds when in close proximity to each other. They can also be individual. Their biology is similar to humans, with an extremely similar genetic coding base. The Vamusk however at this point are able to pretty much exist in a virtual world within their bulb colony and have pretty much mastered mind uploading and various forms of neuraltechnology that allow them to control machines with their mind and all that.
They don't really care to "conquer" earth, instead they just set up colony near a good river in a human sparse region of the world far from any native civilizations and stick to North America, despite doing surveillance of other parts of the world from time to time. I am presuming that the Old World will develop as it had in our world for simplicity's sake (I want all major pre-European history to be the same). The New World however, will be extremely affected by their new alien overlords and their cultures will have to be different.
This "colony" has limits, mainly in that they technically are the equivalent of a couple ruskies camping out in the siberian wilderness in 1000 AD... The real centers of galactic civilization being quite far from our corner of the galaxy. They also are technically descendants of fugitives who traveled thousands of light years and found Sol a convenient place to hide due to its remoteness while still hosting a garden world with drinkable water and full of edible alien life in the oceans (they really like jellyfish and coral, to say the least). They didn't expect such developed natives and so found a somewhat remote, but not too cold place in the world. Africa they avoided out of concern that word of their existence would spread to other parts of the Old World. Australia has a secondary colony as well, but it's comparatively low key and pretty much a glorified coral harvesting plant.
Regardless, the general idea is they settle their main colony by the Mississippi. Their general activity being around the Southern Pacific, Australia, Antarctica and the Americas. Their colonial activity in for the part 1500 or so years has been regulated to these regions. Going off world is something they haven't done outside of needs to make more anti-gravity material on Mercury.
The question, with this information I have given is, what is the earliest point that European explorers would stumble upon the existence of the alien colony? Would Columbus find out, or would some French trapper find out? Would some of the cultural peculiarities of the amerindians give away their existence or would it only take seeing the tether colony itself?