Life is full of suffering. Suffering is part of the process that gets you to take action or make a change, whatever that may entail. Suffer city embodies the place your mind body and spirit lives until a positive or negative change occurs. So basically if you are suffering and you decide to work on your craft, you are moving towards discipline, but until you master your craft you will remain in suffer city.
The same thing can be said about Regret. You sit in suffer city wishing things were better and decide to give up. When you give up you go the other way towards regret where you become a permanent resident.
Suffer city sounds painful and terrible, almost a hell-like place because it is. Look at it like the Wild West. There are lots of opportunities but it is still a dangerous place to be. Danger forces us to take action. Being in Suffer City is not a permament action unless you let it be. Taking action slowly moves you farther and farther away from the city until you reach your destination, which in this case is mastery.
This could also be known as Pergatory or the incubation chamber. Though pergatory seems more like a place you go where someone else decides your fate, Suffer City is where you get to decide your fate.