Setting is in an era when glass would be expensive, in a cold, mountainous region with a lot of goats and horned monsters. I know horn was used in lanterns, but I'm not sure if it would allow through the forms of light that plants crave. Cold/heated frames would be fine, too, if that makes a difference. Would the type of horn matter, and can I vary the thickness, maybe? I know the Romans used some sort of oil cloth, which doesn't seem very transparent.
Amount of horn is not an issue due to number of goats and horned monsters in the labyrinth within the mountain. Mostly the populace eats meat and goat cheese, but there aren't a lot of plants during the coldest months.
Update: The windows shown at 0:55 would apparently serve the purpose nicely, since folks have pointed out the panes don't have to be completely transparent. No Brawndo necessary. https://youtu.be/ZgiC8uG4Kpc