First off, congrats for the choice of illustration,
@Gary Oswald. The alcazar is a beautiful piece of architecture. Having visited it a few years ago, this was a marvellous throw back. I'd encourage anyone who can to go see it (and all of Spanish architecture in general). The interior is even more spectacular.
Second, one interesting thing is that while feudal organisation is a thing of the past, you can still see it swirl in any group, large or small, where you have a charismatic leader able to attract a loyal following. That following usually gets noticed and attracts others in its wake, though usually with weaker links which break more easily.
Then there's the good reason why you don't want to draw too many people after you: it will be easier to call on them next year if you didn't this time (one hopes. In the case of Aquitaine, fractious vassals were a constant pain. One such caused the death of Richard I, after all). Then you probably can't feed all of them in the field and on the way to the field if you call all of them. The crusaders often had a terrible time on the march, until they figured they should get boats (at which point they switched to having a terrible time chartering boats due to chronic lack of money). And if you go crazy and raise peasants as well, you risk not being able to bring in the harvest.
There's one other structure close to the Saxon fyrd but not relying on a monarch to organise them: town militias, especially in Northern Italy and Flanders.