Worldbuilding/history chatter: the Nights Watch from...

The basic problem here isn't fantasy at all: how to keep a permanent security presence on a distant and potentially inhospitable border is an old problem. 1/
The main problem is not how to generate the force, so much as how to keep it on the frontier (rather than either dissolving into the peasantry or marching on the capital). 2/
And you can see this pattern for the Night's Watch: they're called brothers, they take vows, and they serve for life. 4/
Martin is a secular agnostic and so imagines secular societies, but secular societies don't produce religious orders. 6/
They're going to set up a state. And then march on Winterfell. 8/
That exists too and is a subset of the far more common solution to this problem than religious orders: long-service professionals. 9/
But even if you do that, there's a problem: you have to pay these guys and let them retire. 10/
If you do not pay them - and no one pays the Night's Watch - they'll march on Winterfell. 11/
If it was that kind of society, you'd have very different politics (appointed governors, not vassal lords). 12/
Professionals retiring would, likewise, produce a permanent population. 15/
It needs one religion, which the brothers believe in a lot and which binds them together. 16/
In either case, a highly cohesive body of armed men is not going to be politically neutral the way they often are in the books 17/
But they did wield a lot of influence and wealth! 20/
These guys try to sack Winterfell 11 times out of 10. 21/
They just wouldn't be fighting wildlings or the Others. /end
Once you give them weapons, it's a new game and the question is how you KEEP them.
You can't treat them as individuals anymore - they'll act as a group.
They're not going to break off onsies-twosies, they're going to march south several thousand strong.
Even if you win the war that results, this is a failure state.