Okay, this is going to get a bit more complex, as i am trying to find a realistic number for my story. Let’s assume they have a 50/50 male to female ratio. A few important things to throw out there:

1) The original region is 84000 sq miles.

2) They already have the knowledge that they had around 1000AD, including medical, but they were starting from scratch.

3) Monogamous relationships are the norm.

4) When they got there we will say there was 60% forest, 20% arable land, and 20% wasted land. The majority of the forested land had good soil.

5) They begin to build over 1 area which included a stone castle, then in 20 years, a group of them heads to a new area to build another town with a castle, then finally in 20 more years then head to a third. So by this time 40 years have gone by, and there are three towns , each are roughly 100 miles from each other. (Are these numbers too low? Would they need more time before heading to the next location?)

6) In another 20 years, a portion of the population goes to a whole new region that is about 73000 square miles and begins on a 4th town, 20 years a 5th town, 20 years a 6th town. So to recap, 100 years have gone by with no war, but some disease and sickness. These regions do not come into contact with each other again by 400 year mark.

7) War finally breaks lose in the first three territories after 100 years, and and over the course of 300 years it divides in 3 advanced kingdoms which are about 1300AD development, and 7 outer colonies, which have the technilogical advances still being around 1000AD. War is steady the entire 300 years. This includes battlefield and raiding of towns and villages.

8) Around 120 years in, war breaks lose in the other region as well, and goes on for 280 years. This accounted for 5 developed kingdoms, and resources such as iron and stone were easier to come by, so they were a bit more developed than the other region. This also includes battlefield and towns and villages.

9) Initially the average household has let’s say 5 children per, but dwindles down as the economy takes over. By the year 400, the average children per household is 2 who lives past 15 years old in the developed kingdoms and 3 in the less developed kingdoms. This also factors in the deaths of infant children, as I understand was very common back then. Let’s also say that 1/50 births led to the death of the mother.

The more developed kingdoms produced children at a lower rate than the developed kingdoms as there was more structure to the economy, so people couldn’t afford to have as many children.

10) Disease and sickness killed, but not nearly as intense as the Black Death which killed one third of the population.

11) All 4 seasons were involved, including snow in the winter.

So basically what I am trying to figure out is if 1000 or 10000 are a realistic number to populate both areas of my world in 400 years, and give them enough time to build small realistic medieval cities? And also what the population of both of the regions might be. Thank you for anyone who attempted to answer this. Feel free to make assumptions on things I don’t have listed here, but please let me know your assumotions

I’m not sure if I am leaving out any more details. If you need any further information, feel free to ask, and I’ll try to respond efficiently. Thank you again!

Edit: added points 10 and 11 about disease and seasons.

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    $\begingroup$ You're asking a lot of different questions in your post. Can you edit it to focus on a single question. $\endgroup$ – sphennings Mar 14 '18 at 17:56
  • $\begingroup$ You need to be more specific about climate and diseases. Those can boost or kill your population. $\endgroup$ – Alexander Mar 14 '18 at 18:15
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    $\begingroup$ And the Wikipedia article on Medieval demography is not good enough for what reasons exactly? It has nice tables, gives you estimated population numbers by century and region, etc., so you can estimate growth rates and densities. Ah, and your assumption that towns and castles are somehow related is debatable. BTW, 100 miles is a looong distance in Europe. Second BTW, 84,000 sq. miles is a modern mid-sized European country; for the Middle Ages, a typical independent polity would be much smaller; England is 50,000 sq. miles. $\endgroup$ – AlexP Mar 14 '18 at 18:29
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    $\begingroup$ At least as far as the title is concerned, this looks pretty close to What is a reasonable amount of population growth for 900 years? You might want to check that out. Full disclosure: the highest voted answer is my own. $\endgroup$ – user Mar 14 '18 at 18:33
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    $\begingroup$ Consider what this WB:SE question has to say. $\endgroup$ – JBH Mar 14 '18 at 18:39

Given your current constraints...

Almost certainly not

The most important constraint here is #9. Assuming technology equivalent to 1000AD on Earth, 2 or 3 children surviving to the age of reproductive viability is well below replacement fertility levels. Add to that population loss from war, accidents, and the individuals who simply do not reproduce, and your population will likely go extinct over the interval, not expand.


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