So you’re going with fantasy as your genre of choice? I guess that means you don’t need to worry about science.
Just because you presuppose supernatural forces to exist, doesn’t mean that there can’t be science alongside them.
What “No Science” would actually mean
Look at your typical medieval or ancient world fantasy. There’s no science there, is there? No universities, no computers, no scientists with beakers and lab coats. Ask yourself, though: Do they have metalworking? Do they cultivate the land? Do they have buildings?
Just because science might exist in a primitive state doesn’t mean it isn’t there. A world without science would have the protagonists living like animals. Even cavemen have limited scientific development in the form of fire and toolmaking.
Where to begin? With magic
To decide on the role science in a world you are creating, it helps to begin with a look at the magic you intend to give them. While magic and science can (and do) coexist, the degree to which they do so can vary greatly. The combination of the two will provide a sturdy framework for the society you wish to pour into them.
“Necessity is the mother of invention” is an saying that may be just as valid for magic as it is for scientific inventions. What people see a need for, they will seek to fulfill it. If either science or magic fills a role in society well, the demand for the other to address it will diminish. Keep in mind though that societies are not homogenous, and it’s possible that opposing viewpoints might exist as to whether magic or technology is the appropriate solution to a given issue.
For each relative level of technology, I’ll look at the high-magic implications. For this purpose, I’m going to consider “high” magic to be the condition where magic is common enough that most everyone has heard of it and knows of people who practice it. Magic may still be feared, but it’s impact is widespread and undeniable. A “low” magic world is one where magic is much less common. Science will largely ignore magic, and you can assume that roughly earth-like technology would be appropriate to the setting.
Low Tech: Primitive
While it’s not common for fantasy to look into the lives and adventures of neanderthal humans, you can certainly have fantasy set in a pre-agricultural society. Nomadic tribes can follow the herds and make their living off the bounty of the land. They can make use of tools for portable shelters. They may have domesticated or semi-domesticated beasts. Most of their possessions are self-made from the animals they have hunted.
High Magic: The tribe will probably either be led by a shaman or one will be a prominent adviser to the chief/ruler. Their magic will be seen as a shield against the hostile forces of the magic of their enemies, the wanton cruelty of dark gods, or just nature in general.
Low Tech: Ancient/Classical
From Babylon to Rome, society has developed to the point where great cities can be built. Metals can be worked into tools or weapons. Crops are planted and harvested to feed large concentrations of people.
High Magic: Military might will likely be heavily related to magical might for any kingdom. Magic swords and armor can give heroic aspect to ordinary heroes. Used for civilian purposes, architecture may exceed technological limitations, allowing for breathtaking towers and sweeping bridges. Magic may also be used to increase crop yields or reduce the impact of droughts.
Medium Tech: Medieval
This is fantasy’s wheelhouse. If you don’t give any other hints, but mention “fantasy”, this is the image people get (see my blog about customizing your world away from medieval England). Knights in plate armor, castles made built for defense, monks off in abbeys writing out books by hand.
High Magic: There is so much to improve about medieval society that magic can run amok. Nearly anything you allow magic to do will work better than the technological equivalent. Wizards will be respected and/or feared widely.
Medium Tech: Renaissance
Artistic and cultural expansion are the bywords. Sailing ships roam the world, eating away at the sea serpents’ domain and filling maps with land and sea. Astronomers build telescopes to understand the heavens. Gentlemen hobbyists begin to form the backbone of the modern sciences. Black powder revolutionizes warfare.
High Magic: No matter how hard they try, magic is not going to supplant mathematics or astronomy, but may take over some of the applications thereof. Magical compasses might be superior to magnetic ones, or easier to use than a sextant. Magic might find a true rival in the power of black powder, which may set devotees of the two major forms of battlefield influence at odds. There is actually a nascent sub-genre of “flintlock fantasy” that looks at worlds where magic and gunpowder meet. My Twinborn Trilogy explores this a bit as well.
Borderline Med/High Tech: Industrial Revolution
Steam power, railroads, telegraphs, newspapers. The world begins to shrink as transportation and communication speed up. Society has to deal with the beginning of the end of the artisan class and the expansion of the laboring class. In the cities, technology is a major presence in everyone’s lives. Mass produced goods begin to give rise to corporations and brand names.
High Magic: This is a fascinating point in science to bring in fantasy elements. Most of the industrial revolution was predicated on finding a way to do things better, quicker, and at higher profit than the next guy. Any niche that magic can fill is likely to set science off working elsewhere. Why compete with magic unless you can truly do better? Each instance of magic supplanting technology should be considered for its impact on how society functions.
Interesting twist: Magic users might be a part of that disappearing artisan class, as their one-of-a-kind magical solutions are outmoded by mass production.
High Tech: Modern
Again, this is a whole sub-genre unto itself: urban fantasy. It’s a world familiar to us, with even less need for world-building than generic, “this is A.D.1300 England unless I say otherwise” fantasy. You can say you’re in 1960’s Chicago, London during WWII, or 1990’s Tokyo, and people know from there what to expect.
High Magic: This is one that is a rare combination. Harry Potter comes close with their wizards/muggles division, but there isn’t a lot of well-known fantasy out there that re-imagines modern life with a heavy fantasy influence. You can integrate magic as a commodity, controlled by corporate interests, taught at elite universities (and maybe even some of the shady diploma farms!).
High Tech: Futuristic
This is typically the realm of science fiction, but who is to say that a magical society can’t advance beyond what we have today? There is certainly precedent for magic (particularly telepathic/psionic variants) in sci-fi. Why can’t that same thing be considered fantasy with advanced technology.
High Magic: Every classic futuristic device could be re-imagined as functioning on magic, from ray-guns to transporters, lightsabers to starships. Star Wars is an example of this (more so the prequels, where Jedi are more common). It’s more futuristic fantasy than sci-fi.