How would you design a hallway such that any number of people can walk from one end to the other, but only in one direction?
You can use any amount of doors and mechanisms and so forth, but only purely mechanical things. No electronics or other modern technology.
We’ll assume that whatever you come up with can be built strongly enough to resist tampering and brute force.
A couple of large paddlewheels that form the walls and can only turn one way.
Turn style at one end (or multiple turn styles down the hallway)
Also, just having multiple doors that are locked from one side.
The standard solution is the turnstile. Does that not fit your requirements?
Well it’s pretty easy to jump over a turnstile.
ETA: Although the full-height kind could certainly work. That still allows the passing of small objects through the bars, so it’s not ideal for my nefarious purposes, but they certainly stop humans.
Hmmm. How about this. A hallway has a door at each end. The entrance door cannot be opened if there anything in the hallway ( the weight of any people inside the hall triggers a latch holding it shut ). The exit door cannot be opened if there is no one inside the hall, due to a similar latch.
Is walking necessary or just getting through the hallway? If the latter I’d say a steep heavily greased slide. Or even a water slide. I know there are methods of making fountains that predated electricity presumably they could be adapted for this use if the location is suitable.
A really fast running river? A log ride? A fifty foot fall?
Dammit I pondered too long.
A very short hallway, barely more that a door’s width long, and with doors at either end that open only in the direction of travel. This won’t prevent people from traveling against the flow, but it would be a strong suggestion.
A large number of very sharp spikes arranged on sprung hinges. They point in the direction travel is allowed (like barbs.) When people walk in that direction the spikes give way, when people walk in the opposite direction they die.
Hmmmm, sounds like a Monty Python sketch
Build the hallway inside the event horizon of a black hole.
They have this at subway stations here in Boston, at the zoo, and at the doors of two companies I’ve worked at – there are “turnstiles” that consist of revolving-door-type constructions that run from floor to ceiling, but instead of glass panels they have a “wall” of rigid horizontal rods connected to the central rotating post, forming a plane. The center post can only rotate in one direction. You can go one way through the stile, but you can’t make it rotate to go the other way. And you can’t “follow” the stile around to go backwards because there is a “wall” of similar rigid pipes forming a stationary plane that blocks your progress. The rotating rods can pass through this stationary plane, however, because their bars interleave with the stationary ones.
Make the hallway vertical, with people entering at the upper end. 
We have our winner!
Place delicious donuts at the end you want people to head towards.
ooh ooh … and pie … and bacon salt :)
I think we could combine these for maximal effect.
Two words:
Chuck.
Norris.
Put the donuts at the top, that way you’d have an anti-gravity machine.