In terms of modern inventions that could be implemented in the Middle Ages, with the available technology, there are two which seemed most likely to make a difference to medieval people. The first was simple sterilization: washing hands, submersing implements and wounds in alcohol, and boiling. Just taking these precautions could decrease rates of infections as well as keeping mothers who would have otherwise died of childbed fever alive.
The second is gunpowder, or rather, ‘black powder’, which is its earlier incarnation. It is made of charcoal, sulfur, and potassium nitrate or saltpeter (found, for example, in bat guano). It had actually been invented by the mide 1200s, but wasn’t put to broad use in Europe until the mid-1300s, when it was being made on a broad scale for the English crown. To return to 1282 Wales, then, and be able produce ‘black powder’ might have made some difference in the outcome of the war.
Now a printing press would have been cool to have had. I assume that everything that they’d need to make it was there in the 1200s, but I’d have to look that up. But the repercussions . . . more widespread literacy 200 years earlier . . ..
What about the printing press? Would history have been different if Gutenberg had invented his printing press two or three hundred years earlier?
Feeling the need to use all your little plot bits? 🙂