A change in setting era.

Is it set on Earth, like an alternate history sort of thing? Otherwise I see no reason to restrict yourself to how things developed in history.

 

I think that not having a "typical" medieval fantasy is a great idea.

I remember reading The Lies of Locke Lamora and finding the choice to have eyeglasses commonly appear to be...weird. I thought, hmmm, kinda steampunkish? But that's simply because the "typical" fantasy doesn't include them. I've since done a little research and it seems that eyeglasses, albeit without earpieces, were first used in the 13th century! Although they weren't incredibly common until much later.

If you hadn't mentioned the history of the mandolin, I wouldn't have known it. Apparently, it descends from the lute which has a much older history.

I would only suggest that if you don't have a typical medieval fantasy milieu, maybe allow yourself a few other unusual features to let readers know this isn't a single anachronistic trait but is instead a feature of a broader distinction from Earth history. But of course you could revise the whole society to better match the later time period, if you wished. It's really up to you.

 

You can have almost whatever mishmash of technological anachronism you would like in your story, unless you have a technology precede the technology leading to its invention or necessary for its existence (skyscrapers existing before your civilization learned to produce steel...idk, blanking on examples, but these things are actually easy to overlook.) Your world's history can leave a lot to the imagination, though. In fact, the skyscraper thing would work if the secret for producing steel was discovered, then lost...All it has to do is make sense in the mind of the reader. You can make your civilization be extremely advanced in one area like metalworking, but very primitive in another. Maybe one civilization knows the secret to producing guns and gunpowder, but another on a different continent is still in the Stone Age, but understands crop rotation and heredity in breeding and everything they need to be extremely successful farmers. It's all up to you.

(Ruined skyscrapers in a Tolkienesque fantasy setting actually sounds REALLY COOL.)

 

Are you sure it was a mandolin, and not a lute?

mandolins are related to lutes, but as you say are 1800s on, but the lute, is a medieval instrument and is much older again than that. This means it may just call for a few word changes (i.e mandolin to lute) rather than a change of the entire setting.

 

The Shannara Chronicles television show? Although suggesting in any way, shape, or form that this was a "Tolkienesque" setting might be heresy...

 

Are you talking about the real world circa 1500?

 

Technological mishmash...

On my primary world, there are knights in (sort-of) shining armor riding into battle alongside vaguely roman legionaries on bicycles - toting crossbows and short swords. The last battles of the Traag War, a few years prior to the current WIP, were ended with hundred pound gunpowder bombs hurled by catapults. Since then, the legions have been experimenting with explosive crossbow quarrels. The Emperor sends missives across his realm in mere days using a system of semaphore towers, with the operators reading the vanes via telescopes. The printing press exists, but is a flat, absolute monopoly of the government and church. Because the world itself is terraformed (no coal) metal is smelted in either solar forges (focused mirrors, and yes the RL versions of these things DO get that hot) or by burning 'fumar logs.' (alien trees.) And speaking of aliens, well, the overwhelming majority of those races are gone, but they did leave behind a fair number of trinkets that would fit right in with 'Star Trek.'

It's what happens when you play the 'what if' game.

 

Firearms were used in the later middle ages so I don't think that would've been a problem really. If you want basic firearms in a medieval setting I see no issues with this as long as you take some time and do some research about how this will effect things. Odds are that rapid adaptions with this new weapon in mind will take place across society and warfare, such as introducing its use into hunting, self-defense with the use of pistols and other things away from the battlefield.

Ways to limit the firearms' effect, if you want them to exist in a basic medieval setting, would be to either make them very new and/or outragously expensive so that massed unites of soldiers with them have yet to be deployed and they are effectively at this point a fancy rich man's toy.

And you could probably add a great deal of loathing for the users from traditional warrior nobles given how these weapons goes straight against the concept of war as a contest in skill and valor. The kind of loathing that leaves you bleeding it out in a back alley if caught alone on a dark night or tortured horrifically before your execution if they take you alive on the field of battle.

 

Gunpowder Fantasy/Flintlock Fantasy/Muskets and Magic has seen a recent uptick with books by Django Wexler and Brian McClellan (being the two most prolific Gunpowder Fantasy authors so far).

So using a more technologically advanced setting isn't going to be as strange as it might have been a few years ago.