
In the year of grace 1345, as Sir Roger Baron de Tourneville is gathering an army to join King Edward III in the war against France, a most astonishing event occurs: a huge silver ship descends through the sky and lands in a pasture beside the little village of Ansby in northeastern Lincolnshire. The Wersgorix, whose scouting ship it is, are quite expert at taking over planets, and having determined from orbit that this one was suitable, they initiate standard world-conquering procedure. Ah, but this time it's no mere primitives the Wersgorix seek to enslave—they've launched their invasion against free Englishmen! In the end, only one alien is left alive—and Sir Roger's grand vision is born. He intends for the creature to fly the ship first to France to aid his King, then on to the Holy Land to vanquish the infidel. Unfortunately, he has not allowed for the treachery of the alien pilot, who instead takes the craft to his home planet, where, he thinks, these upstart barbarians will have no choice but to surrender. But that knavish alien little understands the indomitable will and clever resourcefulness of Englishmen, no matter how great the odds against them...
For another take on non-Euclidian geometries, I have good memories of Inverted World by Christopher Priest.
Regarding the case for lowtech interstellar travellers: I can accept the idea that there is some symple physics trick to make a jump drive that we haven't discovered, fair enough. Just as in the story, there would also had to be a similar undiscovered trick for antigravity, since otherwise they have no way of getting to orbit.
Can you however build a ship that can resist vacuum and provide life support with 16th century tech? I guess perhaps your species might be able to live in vacuum, needs only a bit of heat and doesn't need to breathe gasses?
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Anyway, I’d like to recommend another book. High Crusade by Poul Anderson[0]. In the first chapter pacifist aliens land in 13th century England and get immediately massacred by the locals. Now the king has a starship.
Of course, no intelligent birds exist, but we can see enough of the personalities of some species to get a sense of how an intelligent bird might act. They'd probably be less tribal, less extended-family-oriented, less self-serving than humans, more aloof and self-reliant, more like a startup founder... but also much more violent and warlike, as no avian however large has the same instinctive reluctance to kill a member of its own species that all large mammals share.
I don't know nearly as much about the _other_ path not taken, the cephalopods -- but I've read enough about the behavior of Humboldt's and giant squid when confronted by an unfamiliar sight to get the impression that there is a reason why nature didn't manage to take it. Unlike squid, octopodes are not psychotic, but they're easily bored and they have a malicious sense of humor. The urban legend of the octopus who waits until the lights are out and the aquarium-keepers have left, then crawls out of the water and goes over to the neighboring fish tank to have a snack before slipping back to his own tank before the morning, has happened any number of times.
Little-known medieval trivia point: if St. Thomas Aquinas was right about a certain speculation, the study of avian and cephalopod behaviors is probably a prudent one. Aquinas believed that if other worlds capable of supporting intelligent life exist, they _would_ be populated by intelligent beings, since God wouldn't let a perfectly good dwelling-place for souls go to waste. The only question, to the medieval mind, was whether such worlds existed (and, secondarily, whether Christ would have come to them separately had they fallen, or whether it would have been necessary to convert them to Catholicism; for more on this mindset, read C.S. Lewis' Space Trilogy and perhaps Poul Anderson's _The High Crusade_).