In a world set in the distant past, People A decides to use People B living a good ways outside the city as their first line of defense and early warning signal. They have managed to indoctrinate People B by breeding out rebellion and supplying them with all the necessities to facilitate a non-scarcity environment via the river running through the cities, along with the issuance of some habit-forming drugs. The kicker is that People B do not even know about People A, which is a designed fail-safe. This is done because People A's population is much smaller than People B's because their procreation procedure has been disrupted, which is why they have the need of "disposable" manpower and to remain covert.

My question is... Could People A keep their existence covert while publicly transporting these goods to People B assuming technology consistent with the medieval period?