Lucifer's Cage
Dean: Why break the seal anyway?
Castiel: You think of the seals as locks on a door.
Castiel: Lucifer walks free.
Dean: Okay, last one opens, and?
– Dean, Castiel, 4.02 Are You There, God? It's Me, Dean Winchester
Without me, you got zero shot at killing Lucifer. Sorry. But... you can trap him. The cage you sprung Lucifer from, it's still down there, and maybe, just maybe, you can shove his ass back in. Not that it'll be easy. You got to get the cage open, trick my bro back into it, and oh yeah, avoid Michael and the God Squad. But hey - details, right? And here's the big secret, Lucifer himself doesn't even know. But the key to the cage, it's out there. Actually it's keys, plural, four keys. Well, four rings from the Horsemen. You get 'em all, you got the cage.
Castiel described the Seals as locks on a door that held Lucifer prisoner. On God's command the great archangel named Michael bound his younger brother, Lucifer, with 600 seals. According to Anna only 66 had to be broken for him to be freed from his confinement.
4.22 Lucifer Rising
Sam unknowingly releases Lucifer from his cage by killing Lilith. Lilith is revealed by Ruby to have been the last of the 66 Seals: "And it is written that the first demon shall be the last seal."
5.19 Hammer Of The Gods
In a hidden message recorded on a Casa Erotica DVD, Gabriel reveals posthumously how Sam and Dean can trap Lucifer back in his cage. The keys to the cage are the rings worn by the Four Horsemen of the Apocalypse.