The purists can argue compatibility or not. 3e folks can talk broken builds and alignment restrictions.
I call too much anal retentiveness.
Is it fun? Is it a cool backstory? Is the player having fun?
Let's say Hrothgar the Barbarian washes up from a wreckage of his Viking longship. By a twist of fate, a local Lord - a pious man and worshipper of a Lawful Good god finds him on shore and nurses him back to health. Hrothgar is thankful for the aid and begins hanging around the Lord's paladin order and eventually accepts the Lord's faith. While he never got used to heavy armor, his primitive 2 handed axe fighting berserker fighting style combined with the direction of the light made him a fearsome bastion of good and order. Able to shrug off injuries and smite those who would undermine the powers of mercy using unorthodox tactics alien to the average knight.
Why not?
This isn't 'Nam, there are rules!
What you're looking for is called a.... Fighter!
True.
But rules are just a framework. Rules are you roll a D20 and add a number to determine success and failure. Rules are you gain advantage for flanking. Rules are you need expensive material components for some spells.
But, if a great player had his heart set on an awesome backstory would you break his motivation and possibly turn him away because of an outdated mechanic that does nothing to improve experience?
Plus, unless you are giving this thing monster HD and abilities (3e,5e) is a multiclass Pally/Barb really a I WIN button at level appropriate encounters?
Personally, I see the "Nights Watch" as either Rangers or Psuedo-Rangers. There are plenty of Night's Watch people who don't possess the necessary courage to be true Rangers. That guy who Jon Snow beheaded was a perfect example. Just a cowardly "Fighter", if Fighter is even the appropriate word.You've seen/read/at least know of Game of Thrones? And the Nights Watch? The military order of defenders on the frontier that can be seen as knights....
Who says an entire "order" is involved? I'm thinking more along the lines of individuals within barbarian tribes.That sort of order is going to be a hybrid of knight and barbarian.