Why a defensive fortification is useless.
There are many reasons to build a wall, you might want to separate a bunch of people like with a gated community or hold up your roof. In this case we're considering building a defensive wall for the fief. You may be thinking intuitively, wouldn't a defensive wall be useless for a bunch of wandering goat and sheep herders?
So let's look at what a wall does. A wall acts as a defensive force multiplier, if there's no one on the wall then obviously your wall is going to having zero effectiveness. Defensive walls for this reason typically turn up in decent sized towns and cities where you can have guards to man the walls and defend against attacking armies. While it's true smaller towns might have walls in their case it's not for defense but rather separation, think less defending against armies and more like a really crappy gated community that only allows the locals in.
Next is what a wall doesn't do. What it doesn't do it project any offense or magically filter out thieves, while this seems obvious those are what rural peasants really need. The most likely forces country peasants are going to come up against are going to be wandering goblins, the occasional wolf and maybe the odd skeleton or bandit. Against those forces you don't want everyone to sit behind a wall you want to grab a bunch of men and a good tracker and quickly hunt them down before they can get up to any mischief.
Even worse if fighting an intelligent foe such as thieves or bandits a wall simply isn't going to do any good, without significant resources going into the wall being watched they could simply get in while not watched or at night or most likely just ignore the walls and go after those outside them. In the case of an actual army such as an orc war-band turning up then those walls aren't going to do much good against such a foe with the forces at our disposal and in fact we and Stirland would have much larger problems to deal with, notably the fact that orc war-bands have reached so far into the Empire without being dealt with.
So what would be a good way to increase the defense of the fief instead? Well the easiest and cheapest option would just to be to make sure everyone has slings and maybe host a yearly competition to test and hone the peasant's skills. Slings are cheap, lead bullets are cheap, stone bullets are free. On top of that slings are quite deadly, they have ranges that are often as good or better than bow and can easily cripple or kill with a decent hit.
By ensuring everyone has a decent sling and encouraging their skills with competition (furthest shot, fastest shots, hardest shots, most accurate shots, whatever you want) every peasant will be well defended against the most likely enemies they will encounter and can band together to become truly fearsome. Also it would likely cost less than 25gp to fund this for well over decade even with competition rewards, free slings and food.
But maybe you want something more impressive. In that case an armory is a good choice, by supplying the locals with some sets of gamberson, chain-mail, and helmet you'll have a strong defensive armament for when they need to fight. A shield, an axe or spear and a crossbow could also be considered for armaments. This would be more expensive but if you really want to go all out then this is the way to go, it's like a wall you can carry with you after all.
