As an avid reader of fantasy (and also as a writer), I’ve seen comments like this pop up every now and again, and they have a tendency to raise my hackles. Why is that, you might wonder?
Well, if you’re basing your fantasy fiction on factual events and inventions that have taken place on our own planet Earth, then anyone can criticise an author for any noticeable inaccuracies. That’s all fair and square, but for the most part we’re talking about FANTASY. Whether or not the worlds we write about are feasible and probable somewhere in the universe where physics might work differently doesn’t matter; what matters is that we as authors have invented them whether they’re purely from our imaginations or possibly ancestral memory.
So here’s the crunch; we’re talking about different worlds entirely, or perhaps parallel/alternate universes. Steam engines might have been invented two hundred years before the 1800s in another universe or alternate timeline! Or, if you’ve ever watched the TV series Fringe, you’ll know about another method of transport that took off (pardon the pun) in the alternate universe but failed in ours.
For an entirely different world based on perhaps the 12th century or even the 17th century, you’re definitely going to need to do a lot of research and keep things as much within historical context as possible (so a 12th century setting with fighter jets probably isn’t going to work), but you actually do have room to tweak a great number of things in whatever way you choose. So, if you want to have cities with perfectly clean streets, you can if you’ve got a means to describe exactly why it’s like that, because in the 12th century on Earth, most streets would be packed full of horse manure, human waste, and rats laden with disease.
However, some of these critics just don’t pick up on the “it’s set in an entirely different world and therefore doesn’t have to follow Earth’s history to the letter” fact. An author may think it’s a different world and doesn’t warrant any explanation, and they might be right to some degree, but by explaining it you’re at least covering your own backside and ignorant critics won’t have a leg to stand on.