IIRC, they have lots of mountains, but the valleys and forests are infested.

It seems like setting up new towns is incredibly difficult, and due to the scale of the defenses you need, you end up making a city. But once it's up, Grimm seem to leave you alone after enough of their own die.

Unless their attraction to negative emotion causes a "tipping point" and then a cascade- one bad day, and a single Grimm's actions can cause enough misery or fear to get every Grimm within a mile to join in, then 2 miles, and so on. It's like if a bear walked into a town, and the fear it caused attracted more bears and other predators. Sure anyone can take out one with a decent shot, but 25+ and you need a platoon. And Grimm are a lot worse than a bear.

Even if the above is true, a history lesson on exactly ''how'' Mountain Glenn got overrun could have done wonders for exposition, and making the Grimm more terrifying. Hammering in how one mistake can wipe out a whole city.