There weren't a lot of missions that led directly to Alterra's Mercenary Hall. In fact, there were only very few official ones. Most missions heading to Alterra were set up by outsiders hoping to lessen travel time for their own people.
Alterrans themselves rarely had to summon mercenaries at all—there was simply no need to. For the most part, those who set up a mission likely received posts of a teammate asking for them to set one up.
It was quite costly, but it was the preferred mode of many who wanted to go farther away and not travel for several days.
However, it had to be said that the Alterrans themselves wouldn't use it if it were just from Ferrol or Bleulle. If it weren't too far away, they'd prefer to walk and train by the way.
It mostly stemmed from the fact that they were a bit stingy. The cost of posting a mission could buy a lot of things in Alterra, after all.
