Re: Low-Tech Kingdom Population Density?
 

Short answer:

Monsters, and only being near-human.

Long answer:

A long time ago there was a war. Nasty thing it was. Some of the weapons from that time remain dangerous; one of those weapons was intended to decimate civilian targets, provide area/resource denial and generate disposable self-directing weapons by consuming the local ambient mana (itself a resource the designers wanted to deny their opponents). It, ah, was more successful than expected. Its' success was made even more unfortunate with the defeat and mass-slaughter of its' creators; everyone still alive, and their descendents, are all identified as enemies by these weapon systems.

And so when the local mana reaches the appropriate thresholds, monsters happen. This is a worldwide problem, mind. This particular kingdom has a very large monster problem due to the nature of their magics; their magics don't consume mana -- thereby monsters happen frighteningly often, in unpredictable patterns and at unpredictable locations. Good thing they have guns! (Heh.)

On top of the semi-regular deaths due to monstrous incursions, their reproductive strategy has been tampered with by their gods when they were placed at their present location. The details of which are complicated, but one can assume they have perfect meditation-based contraceptives available to their women that they must learn in order to become fertile in the first place. Sexual behavior is human-normal but fertility itself is a learned skill for both their men and their women. (... a religiously-mandated skill taught to everyone by the religious class as part of their welcome-to-adulthood rituals. Yes, this has caused problems before. Heh.)

Hunger requires overpopulation, bad land management or bad skill/time allocation at the population level. Overpopulation isn't going to happen due to monsters and fertility concerns; the other two might but aren't intended to be occurring during the time period I'm concerned with. Conflict does occur, both among themselves and between them and the monsters. They've had three kingdom-wide civil wars over inheritance disputes, a handful of plagues, the occasional commoner rebellion, etc ... over the length of their history.

Their history places them as being put there about 800 years ago. Resource depletion should likely be a concern, though it can be addressed through the player-floated idea of enchanted terraformers. I know I want them to have wants/needs they can't satisfy themselves in order to spur their acceptance of a trade route with everyone else once they've been discovered and the other kingdoms thereby discover firearms. (Not that the firearms use gunpowder, mind, but rather an alchemical binary explosive which requires mana-sensitive processes to create. Functions anywhere, but it'll only form under certain conditions that the other kingdoms aren't able to replicate; too many mages casting mana-consuming spells!)

Re: Low-Tech Kingdom Population Density?
 

She wishes mild winters, hot summers and much rain. I've placed them in a pocket of land on the windward side of a coastal mountain range, just south of the sub-tropical climate band. They don't have a dry season, only seasons of "lots" and "enough" precipitation. Average surface temperature for that area of the world is roughly 60 F, with winters averaging 40 F and summers 100 F.

Short, mild winters and long, hot summers with many storms and showers.

Imagine, if you will, the remains of a explosive volcanic caldera large enough that Toba looked like a poprock in comparison. The caldera is being tilted so that its' westmost section is submerged with its' eastmost section being above sea-level due to a plate collision which improbably had both plates buckle upwards rather than one going underneath the other. Its' broken, eroded, bumpy, is rather resource-rich, has plentiful flat patches near the coast, rolling hills near the mountains and a sharp climb in altitude over a relatively short distance nearest the rim where it blurs into into the World's Teeth Mountains.