This was a really fun read! I was a bit confused when Karl saw the metal and glass carriage, and it was a good sort of twist once I realized he was talking about a car.
Seeing a multiverse-ish travel story from the perspective of someone from the medieval times is really interesting! I had fun seeing Karl be amazed by all the futuristic things. It was especially fun seeing how enthusiastic the General is about learning about Karl's world. Having that part of history forgotten in General and Mack's world was a neat piece of world building, and it makes me glad that their job seems to let them potentially explore even more past ages!
Great story!