Assuming you already have, You now have a few basic soldiers with which to populate your dungeon or fortress. All well and good, but you need a few people to add a touch of class; the sort to say things like I say! You there! Fetch my tiffin! Gad, those soldiers are so frightfully common, eh-what-what-what-what-what?
In short, you need some officers. You might protest that, saying you need some knights or nobles. Rubbish. Toffs is all the same.
Now, a quick look through the SRD suggests that an officer is simply a higher-level soldier. Makes sense, right? You go up levels, you get promoted. Sensible enough, but it's not as true to life as it could be. Granted, there are those who are promoted up through the ranks, starting off as privates and making it all the way up to field marshal... but that's not how it happens most of the time. An officer has different training, different duties, and on the whole leads a much different life from the majority of soldiers and NCOs.
Prior to the Great War, most officers are from a privileged background. If they are not from the aristocracy then they at least come from wealthy families, in particular wealthy families that seek ennoblement through loyal military service. They attend academies, the more prestigious the better, and besides their weapons drill, learn tactics, leadership, horsemanship and negotiation. Officers are taught to be looked up to by the rabble and are expected to conduct themselves as gentlemen. By and large (in lawful societies at least) they are expected to fight honourably and to refrain from hoofing the enemy in the nadgers when he's on the ground.
That's the sergeant's job.
When the officer's not looking.