Re: Low-Tech Kingdom Population Density?
 

But there needs to be wet roads for trade, either rivers (and navigable rivers) or else usable coasts. Otherwise moving stuff from place to place gets really expensive, as per GURPS Low-Tech.

Re: Low-Tech Kingdom Population Density?
 

I'm working this from the other direction: It is a player-requested setting element, and so its' design is constrained by the details of the request. One of those constraints is that its' size has already been defined, "about the size of the Yellowstone National Park in our world". Y'all have explained that it isn't reasonable for a TL 4~ isolate to maintain itself within that size constraint.

The constraint isn't flexible, and so I've started looking for ways to make it reasonable. The answer is likely going to be one of the other requested details, "who practice an alien form of magic"; the idea of enchanted 3D-jigsaw-puzzle boulders acting as terraforming devices has been floated for my consideration.

Re: Low-Tech Kingdom Population Density?
 

Would your constraints allow the people to practice a form of agriculture that is drastically more productive than European open-field seasonal agriculture in a cool temperate zone? With adequate water and labour, tropical rice agriculture produces several times more food per hectare per year, and I think there was a system in Mexico that was also far more productive than European practices: chinampa.

Re: Low-Tech Kingdom Population Density?
 

She didn't specify their food sources. I've been assuming an oceanic 'warm' temperate climate, with easy access to plentiful sea food. Rice should be acceptable. What is "champa"? (Google tells me about a Hindu kingdom and that it is a named location in Mexico, but nothing I can find about an agricultural method.)

Re: Low-Tech Kingdom Population Density?
 

Chinampa. The Aztecs built networks of artificial islands on the lake around Tenochtitlan and grew a mix of corn, beans, and other plants. It was quite enormously productive. Arguably, that may have had less to do with the specific technique and more to do with the facts that a) it involved maize and 2) it was done in a near-tropical climate rather than Europe's more temperate zone, but it still did produce a lot of food.

Re: Low-Tech Kingdom Population Density?
 

You make a much stronger case that innovation stops or significantly slows down than that there's significant technological backsliding. Many of the things you mention are about stuff that's being invented, discovered, or developed elsewhere, and then never reaching an isolated England. But there's a difference between that and losing access to technology that's already present.

Which, it seems to me, is consistent with what actually happened in Edo period Japan, although they didn't cut off all foreign trade, and they did seem to have a significantly higher population (estimates seem to be 20 million-ish in 1650).

Re: Low-Tech Kingdom Population Density?
 

Lets' see.

http://irri.org/ indicates that the biological nitrogen fixation due to the flooded fields effect allows stable yields up to 3 tons per-hectare per-crop without requiring additional fertilizers. Crops are harvestable about 120 days after establishment; roughly three crops a year then.

Saethwyr has an arable land value of 3,470~ square miles, which is 898k~ hectares. Assuming no more than 20% of the available land being used for such purposes, that gives 179,600 hectares. At 3 tons per hectare and 3 crops per year, that works out to roughly 1,616,400 tons of rice per year.

WolframAlpha says that roughly a dozen servings of wild rice per day would meet the 2k calories daily requirement. Assuming seafood and other foodstuffs is used to make up the difference in calories, minerals and vitamins ... thats' 1,968 grams of rice per-day per-person. There are 907,185 grams in a ton.

1,616,400 (tons per year) * 907,185 (grams per ton) / 1,968 (grams eaten per day) / 365 (days per year) = 2,041,393 (people fed 2k calorie diets per year).

Hmm.

Ah. Thanks. Looks interesting, and pretty.

Re: Low-Tech Kingdom Population Density?
 

Edo period Japan is the closest thing which I can think of to the situation in the OP, but it had much more land and people. Possibly Tibet or parts of Afghanistan in some periods ... although I would argue that they were not really TL 4, just medium-rich, medium-organized societies with horses, ironworking, literacy, towns, and muskets.

The population of Edo Japan was one reason why I guessed that tens of millions of people would be required.

Re: Low-Tech Kingdom Population Density?
 

What about climate? Wet rice agriculture will produce more than three crops per year, indefinitely, given a year-round growing season. But if there is a winter cold enough that plants stop growing you get only two, and if the winter is at all long and cold, only one.

Also, what about landform? Rice fields have to be level and edged by bunds so that they can be flooded, and have drains so that they can be drained. And they need water supplies with catchments and tanks and aqueducts: if the rains are seasonal water storage has to be huge. The kind of landscape actually prepared to grow three crops of rice per year has had a lot of work done on it, and is distinctive.

Java is fantastic.

It's as close as my memory came to "chinampa" without checking. Sorry.

Re: Low-Tech Kingdom Population Density?
 

I am thinking Malthus here, what keeps the population in check? Shouldn't there be hunger, conflict, outright warfare?

How long has this been going on? There must be a lot of resources being depleted. It is incredibly difficult to create a self-sustaining closed system.