There are several points that strikes me as strange in the original article with regard to what is medieval anyway. Not that I find D&D implicitely medieval (I don't and I haven't met anyone who is convinced of it) but because the author, imho, chose strange things to nitpick.
article said:
The way you advance in a feudal society is to win glory in battle for your overlord. Then he grants you land, which is the main form of wealth. Unless you’re a peasant. Then you can never advance at all.
He criticizing the upward mobility offered to players characters. Agreed, it's totally against the feudal average and even outside of the ambition of many of the men of the broad era but... Heroes are not supposed to be typical denizens. Of course Joe Peasant won't advance socially, he'll also stay a commoner, can't gain XP by RAW and will probably behave more in line with the expectations of the author. PCs are by design exceptionnal, Joan-of-Arc level of social mobility (though she wasn't a peasant from the start), or Alcuin of York or Gerbert of Aurillac level of upward mobility when playing a cleric. Exceptional heroes stand out...
article said:
That’s not at all what happens in D&D. There is no overlord to grant you land. Land, instead of being a form of wealth, is completely free!
I was under the impression that PCs were not taking away a castle from someone when doing that. They were establishing new lands under the rule of the king by taking it from the monster-occupied wilderness. I am pretty sure someone able to single-handedly carve a barony from the Moors in Spain in the XIth century and decide to make it part of Castile would have been welcome to do so. Like what Rodrigo Diaz de Vivar did. Granted, he wasn't exactly a commoner but he was an exiled knight. Doesn't strike me as a close to the establishment.
article said:
If you build a castle in the “wilderness”, you have to clear the area of monsters for 20 miles around. You then gain control of a handful of villages within this area. You don’t have to compete against any other ruler or pay taxes to any overlord for these villages!
You just cleared the area from the monsters. A Saxon is obviously a kind of tree-worshipping monster. You just offed the overlord of the village... to whom could you possibly pay taxes ? Given the relatively low level of administration of the border of the Carolingian Empire, I could very well see a local noble expanding eastward by himself if given the occasion (which was much rarer than PCs can, since the latter can cast fireball).
article said:
There hasn’t even been significant inflation or deflation since the dungeons were built. The richest dungeon treasure hoard, on level 13 and deeper, averages out to about 10,000 GP in coin.
How can the author estimate inflation or deflation if he can't know the buying power of said 10,000 gp in coins in the fallen civilization that erected the dungeons he's surprizingly not bitching about (13 level of underground construction inhabitated with monster doesn't strike me as medieval, but it's the lack of inflation that irked the author)?
The monster descriptions of “men”, “elves”, and “dwarves” don’t suggest that the game is set in a European culture. The types of “men” are Bandits, Berserkers, Brigands, Dervishes, Nomads, Buccaneers, Pirates, Cave Men, and (perhaps) Mermen. Berserkers are a little Nordic in flavor, but are balanced out by Dervishes and Nomads from the “desert or steppes”.
OK, granted, buccaneers and dervishes don't evoke european middle ages, but what is strange about nomads? Huns, Avars, Bulgars were active during the middle ages...