@Wowpeaches I was thinking that he gave the information to his friend in the last panels, which is why his friend's returning home and can't come back.
Oh boy, so much to talk about here. I'll put specific things into spoiler tags, the rest will be general information about the systems of this world.
1) Antrim has a castle. (Was shown in previous chapters) It was used in the previous Antrim War in an attempt by the previous lord to hold off the invading army of Haurelia. He managed to help large portions of his populace escape, whilest he and his entire family were slaughtered. Every. Single. One. The entire bloodline was wiped from existence. As such, a castle is great, but it's a last point of defense and it's in an open area. It can be completely surrounded, which turns it into a place to die, not a place to defend your country. The place fortified by Balud is a large area, but it's only realistically accessible from the East, which means he can limit the number of enemies he will face at once. (Think of the tactic used by the Spartans in "300")
2) Magic is a thing in this world. It describes the ability to manipulate basically the "laws of physics" using your own magic power. (Following stuff is still a general explanation, but I'm not entirely certain on whether the manga will go into detail on this or not. I think it was explained in the LN at this point, but just to be sure I'll put it into a spoiler tag.)
This makes it weaker over long distance, since you need to spread your magic power over said distance. Only very few people with excessive magic power are able to call down something like lightning, as Queen Maria did. (She's by far the strongest magician in the San Juan Kingdom, btw.) Offensive magic became even less relevant when counter-magic was discovered, which can cancel spells a short distance away, thus requiring much less magic power. Body strengthening, on the other hand, cannot be countered and is unaffected by distance, making it superior. This opened up some blindspots for offensive magic, though. You can see that in Balud's first battle against General Ramirez, where Balud turned the ground into sand to make the general lose his balance.
3) This world has crossbows. Shooting crossbows from a bunker is a very efficient usage of said bunker, at least more so than archery. To add to that, a bunker is only useful from one side. This means that if you have to move your defensive line back, then the enemy will gain no advantage from taking over your bunker. The trenches, similarly, are more easily accessible from the defending side, if I remember correctly. This aims at making strong defenses that are worthless to the attacking army when conquered. Also, arrows are really not that useful at firing into trenches. There are better defenses, such as the bunker, but a trench is really cost-efficient compared to e.g. having to erect a multitude of sturdy walls.
4) Antrim is known as "the appendix of Mauricia", because it sticks out and into the territory of Haurelia. It's surrounded on 3 sides [north, east, south] by Haurelian territory. During the previous war between Haurelia and Mauricia, Antrim was the first point of attack, partially because of how vulnerable it is geographically, partially because it was originally part of Haurelia and was taken by Mauricia many decades ago. It's a decent piece of land, but the political situation has stifled investments into it. (Because: war breaks out => Antrim gets overrun, so why invest into it)
5) (This is about the dead guy and is a definite spoiler. They will explain this in the coming chapters, so beware and read at your own risk. It's not a giant "omg this will be the big climax" spoiler, but still.)
Dead guy is an excellent spy and is carrying a method to convey information to others: a magical eye. That stuff is highly unusual and difficult to find, so it's reasonable to assume that they missed it during their inspection of his corpse. Remember, he was still considered an innocent bystander who was taken hostage, a family member of a trusted and well-known barkeep. They couldn't dissect him to their hearts content. The information itself WILL be valuable to the opposing side, because they can now see that >something is happening in Antrim<. That in and of itself is a massively important piece of information, which is why Balud worked so hard to eradicate every single spy entering his territory.
That should cover most of the stuff that people were discussing about here.
@theskrie oh didn't know that since the manga never mentions it. I guess that's LN material or in the future
@theskrie Oh, thanks for the precaution. Though I do not think it really counts as a spoiler since it helps clarify the manga, which has chosen not to discuss it for so long
Modern trenches warfare won't work with medieval army with mostly are spearmen, unless he can train massive amounts number of crossbowmen with repeatable crossbow.
@Kiraidesu when a kid without anyone to return venture at their own, they'll eventually hit dead end.
when rats start mistaken them as dead body and gnaw their flesh, some shady guys showed up give them treatment and food in exchange of their long live loyalty.