"He believed her because here in the shadowy light of the stronghold everything seemed possible. Between the two of them they owned the world and no enemy, Gary Fulcher, Wanda Kay Moore, Janice Avery, Jess's own fears and insufficiencies, nor any of the foes whom Leslie imagined attacking Terabithia, could ever really defeat them."
This quote comes in Chapter 4, just after they have finished building their castle stronghold in Terabithia, the first day that they have conceived of the game. It describes the sense of belonging that Jess feels in this newfound kingdom, where he and Leslie rule supreme, idealized and undefeatable and immortal. He sees it as a perfect escape from harsh reality and not just externals like school enemies, but also his own self-doubts and fears. It offers a ray of hope which he sorely needs as he struggles to make the transition between childhood and adulthood in a way that will please everybody. In Terabithia, he lives by his own standards and according to his own impulses and personality. There, he feels himself to be the person he is struggling to grow into.