Question: I am early in this design but am imagining a fictional world in the age of exploration including classic fantasy tropes of monsters and magic, with renaissance era technology, that is cannons, muskets, clockwork and wacky renaissance war machines perhaps entwined with magic albeit to limited effect or making use of fantastical beasts. I would like the world to stay in this state of technology for as long as possible.

So I indeed ask, What would stop advancements in technology beyond this state? Specifically weaponry?

Answer: 
* **Magic sinks the scientific method.**  

At some point, [natural philosophers](https://en.wikipedia.org/wiki/Natural_philosophy#Natural_philosophy_in_the_early_modern_period) would have stopped *guessing* about [phlogiston](https://en.wikipedia.org/wiki/Phlogiston_theory) or [humors](https://en.wikipedia.org/wiki/Humorism) and started doing real science, using the [scientific method](https://en.wikipedia.org/wiki/Scientific_method). They think about things they see, they develop a coherent theory, they think of *experiments to falsify the theory* and *observations that would be predicted by the theory,* they observe and experiment, and they think and talk some more.  

Except that the world is just slightly magic. Observers influence the outcome of the experiment on a macro scale, not just on the quantum scale. The boiling point of water suddenly depends on how many people are watching, and if they want it to boil or not. At least a little. Black powder is more effective if it is mixed by people who *believe* in it.  

That sabotages the progress of science.
* **Dragons really like to nest on steam engines and blast furnaces.**  

They also enjoy village smithies and charcoal kilns, but not so much that a determined blacksmith cannot discourage them. But a steam engine, that's where monsters from the entire region will congregate. Better stay with water power and mostly wooden construction unless you have an army to protect your steam engine. Which means it doesn't pay to industrialize.
Magic has to be superior to technology.
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The development of weaponry didn't happen in a vacuum, it was in response to battlefield conditions. People stuck with firearms and continued to improve them because they were effective against the defenses they encountered. So if you don't want certain types of weapons to advance, make them ineffective. The obvious solution is to have magic render bullets as harmless with trivial protective wardings, or something along those lines. Something where the magical protection is far easier than the mundane weapon it defeats.

This creates a new problem: why are people using ineffective weapons at all? At what point did magical protections become common in warfare? That's the point where you'd see mundane weapon development stall out. So think about when this happens, and why. What changed to make magical protection commonplace? And how do armies defeat it? Probably with magical weapons of some kind.

So your task is to think about how this society uses magic to wage war, and to devise reasons for why building more advanced machines never seemed like a feasible or worthwhile idea to get the edge over the enemy. I think for this to be the case **magical solutions have to be cheaper, easier to use, and more readily available than the means to mass produce machines**, as well as more effective.

This should be your guiding principle when thinking about how magic is used if you want it to essentially replace the development of modern technology. It should be accessible to people at all levels of society and be fundamentally superior to the technology it replaces. If technology is easier for a majority of people to access and can defeat magic, nobody would use magic, especially in war.