Two settings stand out as particularly important to the story of Catching Fire. One is District 12, Katniss's home, where she has returned in a mixture of triumph and guilt after winning the previous year's Hunger Games. Katniss's participation in the games has changed her irrevocably and District 12 just doesn't seem like the same place. For one, she is no longer living in her family's tiny shack, but a mansion built for the Victors. While it certainly looks nicer and feels more comfortable, Katniss is isolated from everyone else in the District, both physically (her only neighbors are the other Victors, Haymitch and Peeta) and through the circumstances of their lives. When she goes to The Hob, where she used to bring her hunting catches in to trade, she notes that now she enters with coins, and tries to spend them at as many different stalls as possible, to spread the money around to her impoverished fellow citizens.
The atmosphere in District 12 has changed as well. There is a new Head Peacekeeper whose violence includes a severe, potentially deadly beating inflicted on Gale. It's clear that this crackdown is the result of Katniss defying the Capitol by using poison berries to threaten them at the end of the last Hunger Games, which only adds to her guilt. President Snow's visit to her house seals the feeling in her mind that her former home is now dangerous and that it is all a result of her actions.
The second important setting of the novel is the arena for the 75th Hunger Games - the giant, ticking clock. This arena is symbolic of how Katniss has felt since her name was called for her very first Hunger Games: that she will never be totally safe again, that every second is bringing her closer to her inevitable death at the hands of the Capitol. The fact that the arena is a clock with a new horror every hour shows that Katniss is not alone. All the former Victors have a target on their back, with the arena ticking away the hours, minutes, and seconds left before they are destroyed. Additionally, the mathematical precision and logic to the way the arena works suggests a cruelly systematic way of destroying the Victors. It shows that the Capitol is strategic in everything it does, including this new attempt to wipe out the potentially rebellious Victors. It takes the brutality of the Capitol to a new, creepier level.