The setting I'm working on has a parallel world alongside modern-day Earth. This world is Earth-like, though there is somewhat limited magic as well (e.g. people can be transformed into animals and vice-versa), and the overall technology level in the parallel world is Middle-ages (castles, horse-drawn wagons, the usual stereotypical-fantasy setting. ^_~ )
Travel to/from this world is rare, and is through portals which, among other things, transform items being brought through them to the appropriate tech level. So, for example, a shotgun going through comes out as a crossbow, and a chemistry textbook becomes a tome discussing alchemy and potion-mixing. The portals don't affect people's knowledge the same way, though, so something else needs to happen to keep people from, say, learning how to make gunpowder or internal-combustion engines or whatnot in the modern-day world and bring that knowledge across to the lower-tech world.
My plan to keep the world at the lower tech-level despite higher-tech is that certain physical properties in the magic-active world are different enough that knowledge of Earth-side technology won't transfer. Saltpeter + charcoal + sulphur doesn't explode, the Bessemer process doesn't produce steel, spinning a wire loop inside a magnetic field doesn't produce electricity, etc. Also, no or limited coal/petroleum reserves reduces access to concentrated power sources.
So, what foundational technologies would need to work differently, and what resources would need to be limited/restricted, to prevent a Middle Ages-level society from progressing further in technology despite modern-era knowledge being brought in. As to why things work differently, I'm happy to just say "because magic" if needed, I just need to know what to break.
A few I've already thought of:
- Metallurgy (e.g. limited/unreliable steel production)
- Electricity & related tech
- Power sources (limited/no coal or petroleum)
- Chemistry, esp. plastics & composites