This may or may not apply to your problem, but when I reached the Curial Inquisitor title as a Priest, I was able to recruit a soldier (in addition to Hugo and Diabolus) by speaking to the local Inquisitorial Prosecutor located in the church.

I assume every class has their own sort of "hub", with their own relevant NPCs. Given that I've only played a priest, the best tip I can give you is that talking to some NPC will help you. Try different ones related to your class, or who knows? Maybe the same guy does it for all 3 classes.

Hmm. I just finished talking to all characters I found in Glatzburg and no one revealed any kind of information. I think I'll check again as I don't recall talking to a prosecutor, only a Judge and the Curial Inquisitor.

Curial Inquisitor, what kind of rank is that? Is it equivalent to Baron? As Baron when you're thief you can recruit one soldier (it says so in the tool tip during loading at least) so if that's the same case as yours it should be.

Apparently I missed the west part of Glatzburg but I've talked to everyone there as well now but no one gives me any hints about where to recruit a soldier. I would prefer to actually acquire him ASAP so when I finish quests he will level up.