The playground, located somewhere in the Midwest of the United States, is an area that the angels have designated as the sole Earthly Portal to Heaven after the Fall. The portal is a special sigil known as Metatron's Cube, drawn into a sandbox. The playground is typically guarded by one or two angels. After the revelation that there are only nine angels left in existence, Naomi chooses to close the gate until Castiel can locate Gabriel and bring him back to Heaven.
Castiel and Sam arrive at the playground, where they are met by Hannah. Castiel tells her of their need for Metatron to learn more information about the Mark of Cain. Hannah apologizes but refuses to give Metatron up, stating he is too dangerous to be set free. As Sam and Castiel leave, Sam moves to plan B, breaking Metatron out.
After meeting with psychic Oliver Pryce and communicating with Bobby in Heaven, Sam and Castiel talk Bobby into helping them out by opening the gate from Heaven. Castiel and Sam stake out the playground and once the gate opens, Castiel makes a run for it, fighting off an angel as he jumps through the portal. After retrieving Metatron and taking him to the playground, Castiel removes his grace, rendering him human.
Castiel takes Kelly Kline to the playground where she is to step into Metatron's Cube and die upon her soul's ascent into Heaven. Upon arriving, they are greeted by Joshua, who is almost immediately killed by Dagon. Sam and Dean soon arrive, but are quickly dispatched by Dagon, who destroys the Colt. As Dagon goes to kill Castiel, he becomes imbued with the power of the Nephilim child, allowing him to incinerate Dagon, killing her.
After escaping the Winchesters and Castiel, Lucifer and Sister Jo arrive at the playground where they speak with Dumah and two other angels. Dumah immediately orders Lucifer's death, but is quickly dissuaded by Lucifer who reminds her that even when he's weakened she can't kill him. He makes an offer to both create new angels as well as give the angels back their wings, in exchange for instilling him as the new undisputed and unquestioned ruler in Heaven.
Seeking Heaven's aid in locating Gabriel, Castiel enters the playground, angel blade in hand, only to find its guardian, Indra, attempting to get drunk. Indra, who appears to be despondent and suicidal offers, no resistance to Castiel, even offering to let him kill him and say they fought, before allowing him entry into Heaven.
After learning that Heaven may crumble due to the lack of angels from Naomi, she sees Castiel off as he continues his search for Gabriel, locking the entrance on her return to Heaven, until Castiel can bring back Gabriel.