Another thing that’s been pointed out as an issue with a lot of traditional (or perhaps stereotypical) fantasy writing is the contrived and generic English Middles Ages-esque setting. This is familiar from our depictions of Robin Hood or King Arthur but isn’t particularly accurate in terms of technology. For one, the generic Middle Ages-esque fantasy setting tends to omit things like cannons and smaller firearms (which started popping up in the 14th century) while including other things that wouldn’t be invented/developed sufficiently until the 15th or 16th, like plate armor and ocean-crossing ships like caravels and galleons. Maybe it’s part of the larger taverns/pirates/castles obsession that makes a lot of fantasy media interchangeable and bland, because Camelot doesn’t sit well in an age of cannon and no one thinks of pirates rowing galleys, but I digress.
There’s also the larger problem, which is technological and political stagnation. OK, so there are kings living in castles and fortified towns with minor nobles causing trouble here and there, but what about the nation-state or city-state’s internal political structure? Or its relationships with its neighbors? Are geopolitics in your world a pre-Westphalian free-for-all, where feuding kings and princes fight over fluid borders for decades or centuries, or is it post-Westphalian with the rights of states to exist in their rightful boundaries being encoded in international law? Not to say that there aren’t still wars between economic and political (but really it’s still economic) rivals, of course. These still occur but under the premise that there’s a balance of power holding all states accountable for their actions in regards to other states, and that these states are considered equal in the eyes of international law.
If the former, what’s the stimulus for wanting to take over the world rather than focus on regional sovereignty like the latter? Do the feuding nobles hope to reclaim a throne they believe should rightfully be theirs, as in Western Europe after the Western Roman Empire collapsed in the 5th century? In that case, various Germanic petty kingdoms like the Franks, Saxons, Goths, etc fought it out largely amongst themselves for almost 700 years until their holdings began to resemble the countries we know today, but for a time each of them (not including the Normans, who came to England towards the end of all this) touted claims to the political/cultural legacy of Rome. At first, this constant warfare took up where the Romans had left off but later led warring states and principalities into the Renaissance-period arms race between siege engines (trebuchets and cannons) and fortifications (the angle bastion being particularly useful).
Or is there some kind of superiority behind the scenes that impels them to do so? I’m thinking of the Targaryens from A Song of Ice and Fire, who managed to take over all of Westeros with a handful of dragons. The reason they were able to unite the Seven Kingdoms rather quickly (at least in the grand scheme of things, compared to how long it took the First Men to do the same against the Children of the Forest) was that military technology in Westeros had stagnated, or at least couldn’t compete with dragons. This technological advantage is why Harrenhall turned into a BBQ and why, with its loss once the Targaryen dragons were dead, Robert Baratheon (and sure, let’s include Tywin Lannister even though he came in late) could roll in and subdue a dynasty that had ruled a continent the size of North and South America (apparently) for three hundred or so years.
I promise there’s a point to all this and here it is. Technology, not just military but political and economic, advances constantly. Maybe not in the same linear fashion as a video game tech tree but according to the conditions that cause innovation to become necessary for survival. Stimuli like war, for example. Someone like Leonardo da Vinci or Albrecht Duerer would’ve only been a guy with crazy ideas without a prince or king in need of some new architectural design to protect their city or a new wonder-weapon to blow someone else’s city up. If the situation had allowed or rather required, I’m sure either or those men could’ve learned the necessary knowledge to design missile shields or jet fighters but instead, they were born in their time and worked with their time’s technological constraints, albeit brilliantly.
But in a fantasy kingdom where armies fight for centuries or millennia (sorry, George, but you can’t win ’em all) using the same spears and longbows without figuring out something as relatively simple to manufacture as gunpowder, they’re just setting themselves up for failure when somebody else does. The same goes for fantasy universes that don’t take these things into consideration as well.