Thanks for the prompt answer to the Hasty question! Following up on land grants, why didn’t medieval monarchs set aside some land which could be allotted to commoner settler-soldiers like the Kleuroch/Katoikoi of the Hellenistic era ? Wouldn’t this create a semi-professional army directly at the beck & call of the throne & not beholden to vassals & sub-vassals of the throne? Or were there any medieval rulers who did do something like this?

Well, they did. The Roman Empire was doing this from the time of Diocletian and Constantine and kept it up for the better part of the Byzantine Empire. Inititally, a bit part of it had to do with the empire having trouble paying the army in coin, so instead it started paying it in kind, and then in land. Whether we call it the limitanei or the themes, it’s a pretty similar system. 

Likewise, the Anglo-Saxons established the fyrd. Under this system, all freemen had to serve in the fyrd or risk a fine or confiscation fo their land. The problems with this kind of system are twofold: first of all, semi-professionals don’t do very well against professionals, due to the greater experience, better equipment, and superior readiness of the latter. Second, by putting people on the land, they naturally tend to spend their time working the land and don’t want to go off and fight for extended periods, because that would be bad for the harvest. In other words, it had the classic problem that these armies took too long to summon and didn’t want to serve for very long.

And the solution was to use professional soldiers instead and fund them through taxation. Hence, Alfred transformed the fyrd from a slow militia that couldn’t respond effectively to lightning-quick Viking raids into something very different: instead, lords and towns were taxed on the basis of how many hides of land they controlled. Every five hides were to pay for one fully armed soldier in the king’s service (the select-fyrd, who were fully armed and armored mounted infantry who could respond to raids quickly) and provide one man to do garrison duty (the general-fyrd) in the new fortified towns known as burhs (from which the word borough and burgh derive). Now rather than having to assemble and try to chase down Viking raiders, all the settler had to do was man the walls of a fort and hold out until the professional soldier could come and chase off the Vikingers. 

We can see the same phenomena if we look backward and forward. The Byzantines first started to hire mercenaries to supplement their armies, because the settler-soldiers weren’t that good, and the professional Kataphraktoi required a lot of time and money to train and equip, so it was easier to hire an army for as long as you needed it rather than keeping one on the payroll. Likewise, when the Plantagenet kings of England got tired of their slow-to-assemble, don’t-want-to-fight feudal armies, they shifted over to charging their subjects a scutage tax to get out of their military service and used the cash to hire professional soldiers instead.