Points of Light as a setting idea always crops up from time to time and D&D 4E heavily advertised with it. But what always bugged me with that is how it ignores how much interconnected infrastructure (to avoid the term civilization) is required to achieve something like the default technological level in D&D.
Especially when you get to the extreme end of Points of Light where it is literally just one city surrounded by monsters.
Farmland
Medieval farming is inefficient, especially when you base it on European crops and thus do not have access to rice or potatoes. Most of the food grown is consumed by the farmer and his family with only a fraction reaching the market. So even in a city of 5.000 you would need at least twice that number of farmers in the surrounding lands to support it and all land around the city for several miles would be devoted to farms.
If that land is already dangerous and monster infested the city would starve.
PoL settings I've heard about aren't very detailed, but from what I gathered the points of lights weren't literally surrounded by monster. A small, less than a day's walk from the city, could still be safe. Danger would lurk outside the settlement, but maybe not the point of having monsters within viewing distance from the city walls. As you pointed out, much of the crops was used to support the farmer themselves (and, not marginally, to reseed the next year), so the effectiveness of Plant Growth, doubling yield, would more than double the amount of surplus to feed the city, reducing the necessary hinterland. There is also no reason to suppose the PoL was subjected to the DM's "sorry, you're on the wrong continent, you can't grow potatoes" curse that affected Europe. Even going by the low yield of the relatively barren England farmland, 14 acres were enough to support a family in a three-fields system (median of 6 persons), including reserving a fourth of their surplus for seeds. Using Plant Growth, they would double their output, supporting 13-14 persons. Your 5,000 inhabitants city would only need 9,300 acres of arable land, less than 40 square miles (that can be lessed to yield double by a single spellcaster working a month each year. They would need a safe area of less than one hour's walk from the city walls to support the 5,000 person city, less if more efficient crops are available.
And its not only food that needs to be farmed. Flax is a very important crop as you need it for textiles and also would take up large tracks of farmland
The 40,000 inhabitants city of Cologne had a catchment area of 5,000 square km2. Doubled yield would reduce that as well. In a post-apocalyptic setting like PoL, it would certainly force more selective subsistance farming. It would make trade caravans (a staple of PoL settings) more important and justified.
Animals
Animals are also needed to support a D&D level of technology. Not only as beast of burden to even allow all the products to be transported to the city, but also to supplement the food sources and, important for adventurers, as a source of leather and also other products like glue. And those animals also need land to graze on or even crop to be fed in the case of larger warhorses.
With the distance needed from the city, I am not sure much howling to the city would be needed. Pasture would be made easier with enlarged plants all around. Plus, easily available safe drinking water would reduce the need to grow barley, freeing space for pasture very close the the city walls.
Wood
Nearly all energy demands are met by wood and especially for smelting this demand is huge.
For cooking and heating other resources can be used like manure or coal (if your point of light has access to coal) but as coal contains sulphur it is not suitable to make steel which the equipment of adventurers need (unless you coke it, but that is outside the D&D technology level). Even wood needed to be made into charcoal first which required even more wood.
One example of the amount of wood needed I found is that to equip one roman legion you needed 44 metric tons of iron which in turn required 600 tons of iron ore and 5100 tons of wood to make
This week we continue our four-part (I, II, III, IVa, IVb, addendum) look at pre-modern iron and steel production. Last week we prospected our iron ore and extracted it from the ground and did some…
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So when your forests aren't safe no iron weapons for you.
Yes. That's why you dont need weapons. Just go in a battlefield and use the ones widely available from the last great war against the monster invasion that created the PoL setting in the first place. Medieval cities used roman marble monuments as source of lime, reused stone for construction works, so there is no reason the same wouldn't happen with weapons (that you can restore easily with the Mending cantrip). You're right a legion would be difficult to field in a PoL settings, but I don't think such a huge military unit -- the 60 million strong roman empire never fielded more than 30 -- would be idiosyncratic of a PoL setting. It would be "one of the legendary armies of old".
Metals
The other required resource for iron smelting is of course iron ore. And you also need a lot of it as during smelting there is a big loss of mass. Luckily iron ore is rather widely available. Still, we are talking about a point of light. Some other metals would of course also be nice like brass. Bronze is hard to make as it is an alloy requiring two metals which not tend to appear in the same place.
And if you want to pay with gold and silver coins a source of gold and/or silver would be highly appreciated.
You'd mostly abandon coinage and go with barter for your PoL economy. Absurd number of coins lies waiting in dungeon. For regular metals, dwarves probably have not only better mining technology than in the middle ages but better mining technology than ours. Look at the cantrip Mold Earth for the sheer amount of stone you can clear by round to reach the metals veins when building your Moria.
Stone
Want a stone wall? Or paved streets? Then you require a source of stone and a quarry there.
Said cantrip can extract a 5 ft cube of stone each round, easily cut into panes. Stone Walls wouldn't be needed: most cities would be already surrounded by huge, pre-catastrophe walls, when the population was much more important. The PoL isn't in a phase of expanding, it's being regressing into oblivion [barring heroic intervention].
Chemicals
Often overlooked, even in medieval times people know some chemistry. For example tanning leather required several (smelly) chemical processes which caused those workshop to be located away from the city. That ranges from simple "chemicals" like sides to sulphuric acid to some rather exotic components like gallnut (the remains of insect larves incubating inside tree bark) to make ink which is rather important to wizards. Other required resources was gum arabic and and iron vitriol. Paper and parchment on the other hand are made by animal hides or out of flax (which also required chemicals).
And there are probably many other things I forgot like having access to tar when you want to have ships, etc.
The point is, medieval technology, and with it the technology in D&D, is already so complex that a point of light settlement can't support it, even if that point is the size of a kingdom.
I don't see PoL being dangerous to the point of having dragons sleeping just outside the city walls. They are designed to reflect the supposed feel of strangeness and dangerousness of going into the
saltus outside of settled area, a fear of "civilized people" when confronted to a dark forest... but not necessarily more common than that. You
could encounter a dragon or a violent fey, it would be assured death but it's not necessarily more common than encountering a bear in a real life forest. It would be the feel of a hobbit (PoL: the Shire, Rivendell, the Rohirrim encampments) traveling the wider world. Most of them wouldn't cross the path of Nazguls.
