In general local towers or forts would be watched by local troops, the men at Sparguard are all local men of that area. In the south there are no local men to man towers so there is only a very few (right now there are only two and one is a abandon ruin I plan on turning into a haunted adventure hook at a later date). In the North Sparguard was only a example of the many watchtowers, the problem is that it has been so quiet for so long in the North that they are unprepared for any real trouble from that direction.
On to the Rangers. The Rangers don't have patrols per say they just roam around looking for trouble, they will not catch everything and it is quite possible for Orcs to cross the river and raid into the North but it is a very hard river crossing and and they don't go very far away from the safety of the swamps. The farther North you get (closer to the mountains) the more trouble you will find but there is very little that can be done about that. There are less than 20 Rangers and the deal with those leaving the Winter Storm is a plot hook, it hasn't happened yet and it will be a big deal (and maybe trouble) when it does. Rangers have to be above average as they fight against the odds, they have the judge power so they can basically execute those who are breaking the law without having to be burdened down with prisoners. The system is brutal but it has to be, to do it nice I'd need 100 rangers easy, that won't happen, so fear, randomness of patrols, intese training and skill become "Force Multipliers". In the swamp they are basically fighting a gorrilla war and keeping the tribes off balance and fighting each other. In the North they do what must be done to keep the peace and keep trade flowing. That is one of the problems with Outland Ranger loyalty they have the power to execute people on their own judgement, they have to be fair, honest and loyal to the city(or at least to the laws of the city).
Ok here is a good example of how the North works, we will go back to the Silk Fish poachers. The penalty for poaching silk fish is death, when silk fish poachers are found they are killed. Fear of this keeps poaching to a minimum, you might get away with it for months but eventually it will start raining arrows down on your camp and you will be cut down where you stand. Two Rangers could not take out 10 poachers head on, so they kill them from ambush and harry them until they are dead or flee. The surviving poachers spread the fear.
The system is a bad one but it was designed for plot hooks not efficiency. I want there to be a wild feel even in the North, this is a frontier. Too many Outland Rangers and it becomes too safe and if it's too safe what do the adventurers do? Your never truly safe, even in the city.
The Ranger breakdown is very workable, remember that just becuase they are called the Outland Rangers doesn't mean they are "Rangers" though, you might have some that have no Ranger levels at all. A straight out rogue with a little bit of wilderness lore would be ok in the North as would a straight out Bard and it would be possible to get Clerics and Palidins that ride as Outland Rangers. A Barbarian could work ok in the South as could a fighter with very high Hide and Move Silently skills and the track feat. Arcane Archers would work good in either and a Druid is a good fit in the south. Outland Rangers is the title of the organization because I couldn't think of another word that would fit where Rangers is. Anybody that could follow their code and is willing to stand up for what they believe can join and fight for "the Law Beyond the Wall". The more that I think of it a Paladin would be a perfect fit as a Northern Ranger. Hmmmmm......