What keeps the alien menace from Earth is the dreaded molecular oxygen.
"Pure oxygen; damn it!"
"Oh no, how many parts per million?"
"This doesn't make any sense!"
"Well? What's the reading?"
"Two hundred and nine ..."
"That's possible to ..."
"No. Listen. Two hundred and nine thousand, four hundred and sixty."
"Surely that can't be right, everything would burn."
"We would burn, everything there must have adapted."
"Amazing."
"Wonders never cease."
"They still speak the same language as everyone else, I presume."
"Naturally, though only in the dominant caste."
"Project Shakespeare was not entirely successful, then."
"Surprising it worked at all with that Oxygen level."
GURPS works for grim & gritty melee and guns, but you'd have to wing the heroic romance. Something I always suggest is Dungeons & Dragons, edition depending on how weak you want the armies to be compared to the heroes.
As for the fake science c1889, the periodic table was a bit empty, but they mostly knew what they were missing. They could read spectral lines in the planet's atmospheres and see what gasses they were composed of. They knew the distances to the planets. They knew space was a hard vacuum. They couldn't tell what was on the ground, though they could make a good guess about temperatures and pressures, and surface gravity is easy, thus the jumpy superhero stories. The greater universe is a mystery, though our sun is the same as the other stars, all the many thousands of them.
Gas masks and air filters are wonderful new technology though. Electrified palladium has wonderful properties for making pure air, or something like that, and so light weight.
They did not know what made the sun so hot, nor the earth's core stay warm, the age of the earth was very old as most science required but the source of energy was a great mystery. Newton's laws have known problems, and Maxwell's equations disagree with it, but they must both be right, so that's a mystery too (magnetic engines propelled by great ground tower emissions, by Nicola Tesla, FTW).
There's telegraphs and player pianos (putting professional pianists out of work, who are suing for copyright infringement. They lost). Organ grinders are still awesome for the poor, with vaudeville, circuses, and freak shows for the middle class. Eugenics is wicked popular and getting your brain-pan measured shows how much of an intelligent man you are, and how stupid poor people and women are. Blacks and Asians both evolved from different kinds of apes to Europeans, which is why they're inferior, and why mixed breeding is a sin before god. Most folk believe
only Blacks and Asians evolved from apes, and white people are therefore still god's creation, though science strongly disagrees (you'll note, that hasn't really gone away in a lot of places yet, and dodging the eugenics movement might be an idea).
Mendelian genetics is brilliant and obvious in retrospect (he fudged his data pretty hard, but people didn't prove that for a long time and he was right so who cares, eh.) and animal breeders have gone crazy with it.
Plate tectonics is still sixty years off, the sedimentation and erosion of rock layers is something to do with the great ice ages, maybe, but dinosaurs were great sleepy lizards living in shallow seas in the heat of tens of millions of years ago, before any mammals existed. Before them were insects bigger than men, likely still found on more primitive worlds, life used to much grander, until we were given domain over it by God, as His most perfect creation.
Radio's about ten years off, with X-rays and rail guns. Motor cars are quite popular, powered by steam engines or lead-acid batteries, with the electric buggies being most successful in the rallies by far. Little iron battleships are the big new thing in military tech, along with the Maxim gun, good for keeping the natives at bay. First class men serve in the cavalry, with pistol and sabre, possibly good steel armour. Small electric-motor hydrogen-lift airships are a bit prone to winds, but useful for military observation. Trains are useful to get things to the nearest canal, where it can be floated down to the nearest ship loading dock (thus the idea of grand canals on Mars, even if just as a relic of a lost world), though long rail is under construction where canals are problematic.
Oliver Twist is an everyman story of your parent's day (so they force it on you in school), where Mark Twain writes for your generation. Orphans and wild boys are still everywhere, parents lost to the Cholera (news! it is water borne!), or the Fever, or the Dropsy, or the Consumption, or the Gripe, or the Blood Poisoning, or just in the mines and factories like everyone else. Disease is known to be caused by germs, and antiseptics and autoclaves are making the hospitals much safer places to visit (most survive them now). Microscopes let doctors see roughly what type of bacteria killed their last patient, and now they wash their hands afterward.
Great public debates are popular events, particularly of politics and who we should invade next, for their own good, and also which science God most disapproves of today. (Most people who are weirdly ignorant of science in the modern world are blindly following the result of some grand debate won by the factually incorrect side in the late 19th century).
I even looked most of that up, but now I'm bored. Also, no links inserted, so never mind.
PC, SJW, anti-fascist, not being a dick, or working on it, he/him.
