What I really need are day-to-day technicalities for minor laborors: The equipment (oxen, plow, how it's hooked up? someone leads it, how fast do they go?) to plow a field, methods to sow seed (scatter it by hand?), how the plants are harvested and threshed, what a weather-proof storage area would be built out of.
And, of course, things that I wouldn't even consider thinking about because I have never been a medieval agricultural laborer. The only frame of reference that I have is small vegetable gardens.
Due to fantastic details of the story not necessary to go into, all people on this planet die by 23 or so. Working out population growth/decline is a project for another day, but this means, for this question, that a lot of labor is going to be done by people not at full physical strength. How much, really, can be accomplished by children 6-13 or so?
This whole excessively-early-death thing has been going on for about 500 years, and so there hasn't been much in the way of technological advancement. They should probably have this basic farming thing down pretty well. Things like the fields being cleared and whatnot could have been done earlier, or just with fire.
There is not much in the way of metalworking besides very bare essentials.
The kingdom in question is approximately 40 - 30 N latitude, bordered in the north by mountains and in the south by an ocean. The climate varies, but what crops might be able to do well in these areas? I can do additional research later, I just need to have a basic idea of what would and wouldn't work. I've crossed out rice as an option as too labor-intensive.
I will be forever indebted towards you.