Low-Tech Kingdom Population Density?

[Turhan's Bey Company]

I'm late to the population density party, but there's an article in Pyramid #52 which covers land use and support of urban centers. You can use that to figure out population densities in a GURPS-friendly way.

I suspect you'd need a population of several millions, if not tens of millions, to do TL4. In addition to encompassing a great many craft industries, those industries imply a lot of specialized extraction and processing industries, which in turn imply the need to cover a lot of space and to be supported by a vastly larger population. Moreover, without a large and well-populated setting, some of the hallmarks of TL4 civilizations (elaborate architecture, various navigational developments) won't be practice, and since these are things which are generally transmitted by what amounts to oral tradition rather than by reference books, they'll vanish if they don't get used.