First, Castiel only remembered fragments of the previous years of his time on earth. They were flashes of images, feelings, and sounds. He wasn't sure about what caused him to forget such a big part of his life. It's not like angels could have amnesia. It was unheard of, yet there he was, trying to gather what was lost. The torture felt like an endless path of agony and confusion. He was unable to decipher if he was still in the room with the Winchesters and those other survivors, or if he was years back when humanity was created. His memories clashing together made no sense in his delusional brain. For a moment, he even forgot how to speak... or maybe he didn't, he just spoke in something that wasn't English. It was like an eternity until he was able to focus on something other than the pain.

Slowly but steadily, he started to retrieve what was once lost. First, there were the memories of his time before the apocalypse came to Earth. He remembered when he came to Jimmy Novak asking for assistance, when he met the Winchesters for the first time, and when he chose to fall off heaven for them. Those were the easy memories, because when he started to gather what happened after pestilence released the virus, things became a mess. He had a big fight with his siblings about staying down to protect humanity from Lucifer. But given that Dean Winchester did not accept to become Michael's vessel, none of the other angels were interested in Earth anymore. They all left and Castiel stayed to try and fight back. However, before doing that, he needed to find his friends and the only weapon capable of harming Lucifer. The Colt. His search for the gun took longer than he expected. For some reason (perhaps shame, or not being able to stop pestilence, or not realizing what the angels were doing when they broke the seals), he didn't want to meet the brothers before having the gun in his hands. He needed something to assure them that he was on their side and useful. Not that the brothers needed reassurance, but he didn't know that. When he finally managed to get his hands on the Colt, he was met with a powerful demon that messed with his head in exchange for the gun. He wanted to know a few secrets from the angel that he extracted just the way they were torturing him to recuperate his memories. The process left him with a messed-up head and a gun, alone in the world. The most important thing he managed to remember from it all was to find the Winchesters and give them the Colt.

Now here he is, with the brain scrambled somewhat back in place, the weapon to kill Lucifer, and the notion of how to find him. He just needs some last ingredients and his mission will be successful. Crowley has to be summoned.