Castiel and Crowley are from two separate, rivaling species and do not meet until the events of the Apocalypse in season 5. Their relationship is mostly due to their mutual connection to the Winchester brothers, by whom they are often called in for assistance.
Castiel is unaware of Crowley's existence until spotting him in Abandon All Hope..., where he witnesses him do a deal. Castiel also told Meg about Crowley's views of Lucifer destroying the demons. This may be where Lucifer learned of Crowley's defiance.
Castiel and Crowley don't meet formally until the latter arrives at Bobby Singer's house in Two Minutes to Midnight, revealing that he made a deal with Bobby in exchange for the location of the Horseman Death. The two do not speak to each other, but remain aware of one another all throughout Crowley's participant in stopping the Apocalypse.
After returning to Heaven, Castiel found himself in conflict with Raphael. Unable to fight back, he nearly sought the help of Dean, but was reluctant since he believed Dean needed peace for once. Crowley took this opportunity to offer his own help, by promising Castiel an abundance of souls from Hell. Castiel used these souls to declare a civil war against Raphael.
Later on, Crowley and Castiel began working to open up Purgatory for even more souls. This alliance was to be kept a secret, and when the Winchesters became involved, Castiel pretended to fight against Crowley. The most prominent example was pretending to have burnt Crowley's bones, leading everyone to believe Crowley was dead.
However, Eve was well-aware of Crowley being alive, and told the Winchesters this. This led Sam and Dean to begin suspecting Castiel. Although Castiel did his best to hide his tracks by killing demons such as Ellsworth, a single slip-up drove the brothers and Bobby to trap him in a ring of holy fire and interrogate him. The alliance is confirmed and Castiel tried to justify it but was unable to get them to understand. Crowley then appeared to save him and Castiel had his friends escape. Crowley notes that Castiel had been keeping this a secret so the brothers would see him as noble, and by doing so, Castiel could convince himself that he indeed was.
Crowley was also becoming unimpressed with Castiel's inability to betray the Winchesters, and blames Eve's death on Castiel.
Castiel himself had never intended to remain loyal to Crowley, and simply wanted to restore Heaven. He proved this by turning against Crowley and forcing the demon to join sides with Raphael, who also prevents Castiel from killing Crowley. The two cause Cass to flee only for Castiel to have tricked them and already obtained the souls from Purgatory.
Crowley was aware of Cass' deeds and fled while Raphael was left at Castiel's mercy. Before killing him, Castiel told his pleading brother that he had plans for the demon.
During Meet the New Boss, Crowley knew that the soul-empowered Castiel wanted him dead, but when approached by the angel, he was offered a chance to remain alive since Castiel wanted to keep Hell in order to keep Michael and Lucifer trapped in their prison.
Castiel's tenure as the new god was cut short by the Leviathans he had unwittingly consumed, leading to his death and the unleash of the beasts upon the Earth. Crowley attempted to work with the Leviathans, but their leader, Dick Roman, did not wish this.
This drove Crowley to side with the Winchesters again, by offering his blood to fuel their weapon. It was during this encounter that Crowley met the newly-resurrected Castiel. He clearly expressed his desire to kill the angel, but chose not to since he saw that Castiel had lost his mind, to the point where he offered Crowley honey.
Castiel himself would ultimately be sent to Purgatory, along with Dean, allowing Crowley to kidnap Kevin and Meg and leave Sam all alone.
Upon Castiel's return from Purgatory, he was under the command of Naomi, who wanted him to obtain the angel tablet. In their first meeting, Crowley was amused to see Castiel again and that he regained his sanity while the latter demanded Kevin Tran and threaten the demon who complied.
Crowley also sought after the angel tablet, and went as far as to torture the angel Samandriel for its location.
Castiel and the Winchesters, along with Meg, traveled to Lucifer's Crypts to obtain the artifact. Crowley arrived shortly after, but he and his demons were stalled long enough for Castiel to run away with it.
Nevertheless, Crowley hunted Castiel down and saved him from Naomi who was also hunting the latter. Having captured Castiel, and shoots him in the gut using bullets made from a melted-down Angel Blade, severely injuring him. Crowley had correctly deduced that Castiel had the angel tablet inside of him, and painfully removes it through the hole his bullet made.
In his weakened state, Castiel manages to escape when Crowley leaves him with a single guard. Later, desperate to save Sam from Gadreel, Castiel works with Crowley to allow them to communicate with Sam, and have him force Gadreel out (having gotten the idea from Crowley's torture of Samandriel). As part of the efforts, Castiel had to burn off Sam's anti-possession tattoo so Crowley could possess him.
Castiel attempted to rescue Crowley with Sam and Dean's help, but failed to do so before Samandriel blurted out the information Crowley needed. Castiel continued to hunt down the tablet, eventually finding out that Meg was in Crowley's custody and had not given up the tablet's location. The plan to use Crowley to save Sam succeeds, and Sam expels Gadreel. Afterward, following Dean's lead, Castiel lets Crowley go as part of their deal.
A disappointed Castiel then began working to do the Heaven's trials, while Dean and Sam did the Hell trials, much to Crowley's chagrin.
Crowley was subsequently held captive by the Winchesters to use for the third trial, and despite them forfeiting the trials, he remained their prisoner. At Castiel's insistence, Crowley was asked to help with getting rid of the angel Gadreel, who had been given permission to possess Sam in order to heal his injuries.
The betrayal of Gadreel and his own frustration led Dean to side with Crowley, something both Sam and Castiel were not happy about. For the remainder of the season, Crowley and Castiel did not interact.
When Dean, as a demon, became a problem for Crowley, the King sought Castiel's help in taming Dean. This was achieved by rescuing Castiel and Hannah from the vengeful angel Adina, and providing the weakened Castiel with Adina's grace. He told Castiel that he owes him and will cure or kill Dean to return the favor.
They continued to help each other when the Mark itself proved problematic. Crowley captured his mother and forced her to remove the Mark, while Castiel mostly stood by. However, upon completing the spell, Rowena cast the Attack Dog spell on Castiel, in order to kill Crowley.
The demon managed to survive the encounter, however.
Castiel's recovery from the spell forced him to stay indoors for a majority of the beginning of the season. It wasn't until Castiel met with the Darkness and was carved up with a message, that he met with Crowley again.
Castiel arrived in Hell with the Darkness' message, just as Dean, Rowena and Crowley were preparing the spell meant to send Lucifer back to his Cage. Upon hearing Sam was in danger, Castiel joined Dean in confronting the archangel, despite Crowley's protests.
As with the Winchesters, Crowley was unaware of Castiel's ultimate decision to become Lucifer's vessel. After the Winchesters left, Crowley was approached by what he believed to be Castiel, until the angel prompted him to "guess again".
Crowley spent weeks being tortured by Lucifer in the guise of Castiel. This did not affect nor change his opinion of the angel, but when he finally escaped and got his hands on a Hand of God, he was willing to kill Lucifer even if it meant killing Castiel as well.
In Hell's Angel, at Sam and Dean's insistence, Crowley agreed to expel Lucifer first. When the angel suppressing sigil failed, Crowley was forced to enter Castiel's vessel to speak with him directly.
Inside Castiel's mind, Crowley discovered the angel to be under a sort of trance, uncaring about the outside world. Crowley deduced that this was Lucifer's doing, and urged Castiel to "snap out of it". He was unable to convince the angel to do anything, and was soon approached by Lucifer. In the ensuing attack, Castiel seemed to only care for his television.
Crowley meets the real Castiel again in Alpha and Omega, after Lucifer had been expelled. The two do not interact, but stay within each other's presence until the conclusion of the final battle.
They meet again in The Foundry. Castiel is unimpressed with this unexpected reunion, but their mutual desire to find Lucifer and send him back to his Cage leads to teamwork. They ail however, to find the archangel due to Rowena's intervention, having banished the archangel elsewhere before they could arrive at the former's location.
With the help of Sam and Dean and Vince Vincente's bandmate Tommy, Castiel and Crowley discover Lucifer's latest plan. Although Castiel finds it unfortunate that he has to work with the demon, he and Crowley nevertheless join forces and try to defeat Lucifer before he can murder the fans gathered at a concert. They are both beaten down and left badly injured in the aftermath of trying to foil Lucifer's plan, though they succeed as he is forced to leave his vessel.
During First Blood, Castiel sought Crowley's help to find the Winchesters who were captured by Secret Service. Crowley didn't know the exact location though assured Cass that they'll be back. Castiel later tried to get Crowley's help again but Mary Winchester told him not to though took it back after seeing he brought Mick Davies and Arthur Ketch (the people who helped take her sons) for help.
In Stuck in the Middle (With You), when Castiel is mortally wounded by the Lance of Michael, Crowley appears on the scene. He reveals info about the demon they are facing and admits to the Winchesters that he's beginning to grow fond of Castiel after seeing his condition. He even goes as far as to destroy the Lance in order to save Castiel, despite wanting to use the Lance against Lucifer.
Crowley sacrifices himself as part of a spell to close a tear in the continuity so as to trap Lucifer, while Castiel is killed by Lucifer using an angel blade when he tries to protect the Winchesters.