Surprisingly close to a standing army, when looked at from a certain angle. Only that it's not taxes going to the central authority and then getting paid to warriors, but warriors getting assigned an area to tax, and keep their cut as compensation for their military readiness.
(of course when looked at from just a slightly different angle, it's also indistinguishable from a protection racket)
Terry Jones' documentary of medieval times made the point that the nobility was pretty similar to an organized crime family. Of course, he wasn't aiming at scholarly accuracy but trying to give viewers the flavor.
It is not an incorrect comparison, superficially it is hard to tell the difference between organized crime and the legitimate government, they both provide the same "services", protection for a portion of the profits.
There is a sort of equivalency in a corporation and a government, I would even say a corporation is a government, it is the ruling apparatus for a group of people. Organized crime is a corporation that is trying to operate outside the law of it's parent operation.
I like to joke that the reason the government is so hard on organized crime is that they don't like the competition.
But really organized crime should be suppressed because it is a brutal warlord style government.
(of course when looked at from just a slightly different angle, it's also indistinguishable from a protection racket)