Since I left off my explanation of infrastructure distribution on the geography of my setting, I've been running characters in the setting and continuing to refine and develop it. I've also experienced an increased interest in playing AD&D 1e according to the rules. Toward that end, it has been helpful to overlay a grid of hexes 1 mile across and to each ascribe one of the terrain types referenced by the rules. This allows me to easily track character movement on the map and know the specific terrain type in which they find themselves when a random encounter check is due.
I accomplished the terrain assignment programmatically, in similar fashion to the 6-mile hex types. I also opted to conform the settlements to how the rules would create them, rather than trying to construct more historically accurate demographics. I wanted my setting to align with other games which follow the rules. I also recognize that the forces of nature in a fantasy world as dangerous as the one which the game describes would probably have a dramatic impact on human demographics.
Appendix B of the Dungeon Masters Guide provides table for randomly generating terrain with settlements, castles, and ruins. The guidance For using these tables is for checking hexes which are 1 mile across or larger. How much larger isn’t 
specified and so one can create a setting compliant with the intentions of the rules of varying population densities by deciding how large each hex to be checked for a settlement should be. Each roll on this table averages 411.195 people. Most of the time, 83%, it’s none, but 1 in 100 is a city of ten to sixty thousand souls.
In my Erin 1478 setting, I’m using the geography of Ireland with a total human population of about 800 thousand. Spread over the island’s 32,595 square miles, that’s a density of about 24 ½ people per square mile. Following the guidance in Appendix B, the inhabitation table should be consulted for hexes each covering an area of about 16 ¾ square miles (that is 411.195 / 24.544). I will not, however, be consulting this table to distribute any settlements larger than a thorp. Since I’m using actual geography, I can use actual settlement locations as well to ensure that the siting of these settlements make some kind of sense. I will use this table to distribute single dwellings, thorps, castles, and ruins later.
For the cities, towns, and villages, what I want to know is how many this table would generate of each. So the first step is to figure out the percentage of the population which would be in each category of settlements. I concluded that the table generates 411.195 people per roll because each roll has a 3% chance of indicating a settlement of 6 ½ people on average, a 2% chance of a thorp averaging 50 people, and so on. The cities, at 1% chance for an average of 35,000 people contributes 350 to the 411.195 for each roll. As such, in the game’s implied setting, about 85% of the human population lives in settlements of this size. Since about 800,000 people live in the setting, some 680,900 of them live in cities. The game’s cities average 35,000, so there will be 20 cities in my setting. Following similar logic, there are also 20 towns, 39 villages, and 39 hamlets.
Taking the names and locations of the 119 most populous settlements in modern Ireland, I then assigned each of the 20 most populous a setting population of 10,000 – 60,000. I did the same for each category of settlement down to hamlet. The thorps and single dwellings will be placed randomly or when impled by events in the game.
Castles ought to show up once per 558 square miles or so, and so about 58 of these are expected across the island. As characters cover previously unexplored hexes, that’s roughly a 1 in 6 chance, 1% of the time for each 1-mile-across hex (0.866 sq mi). Ruins are two-thirds as common, same as villages and hamlets, a 2% chance per 16.7536 square miles, that’s about 39 on the island. The chance for ruins will be 1 in 8, 1% of the time. My procedure is to roll 1 – 24 on a 1 in 100 in each 1-mile hex, where 1 – 4 indicates a castle and 5 – 7 indicates ruins.
The result is this:
