Cersei Lannister if nothing else, was cunning and ambitious. Much like her father Tywin Lannister, she constructed plans without the greater good in mind, and with the goal of the Lannisters coming out on top, no matter the consequences.
More than most characters in Game of Thrones, Cersei well and truly played the game, and in the end had many fans of the show rooting for her just because she was so good at what she did, plotting to enact revenge and to ensure her house thrived.
Sansa's Letter To Robb
In an attempt to reduce Northern uprisings and tensions from Robb Stark, Cersei persuades Sansa Stark to write a letter to her brother to bend the knee to Joffrey in exchange for the release of their father.
Although Cersei's plan ultimately backfires and war ensues as Joffrey orders the beheading of Ned Stark, Cersei was sneaky in her attempts to ensure the loyalty of the Starks. By feigning care for Sansa and the appearance of doing what's best for the realm she was able to get Sansa to write a letter to her brother that would be later used against her.
Putting Qyburn In Favor
Qyburn was expelled from the Citadel due to his experimental practices with medicine and human subjects.
Desipite Qyburn's reputation, Cersei begins to favor him and even goes as far as to install him as her hand. The installation of Qyburn in court led to The Mountain becoming indestructible and invaluable to Cersei's protection. Cersei showed that she had the most power by gradually raising Qyburn's station as he had been shunned by his powerful peers previously.
Continuing Her Affair With Jaime
Even though they were twins, Cersei and Jaime's love affair lasted for decades, even resulting in the births of three children.
Cersei was sneaky and determined to keep their relations under wraps, with very few people knowing the true nature of their relationship, or living to tell anyone about it. This is Cersei's longest running plan, one that she only reveals after she is a ruler in her own right. However, it is not a plan that Cersei executed alone, with Jaime also making sacrifices to ensure their relationship remained a secret.
Her Rivalry With Tyrion
Despite Tyrion also being her brother, Cersei did not feel as warmly about him as she did Jaime. Cersei blamed Tyrion for the death of their mother as she died giving birth to him.
Throughout the series Cersei sneakily tries to have Tyrion removed from any power or favor, along with trying to get him killed. Cersei enacts various plans to hurt Tyrion without him expecting it, capturing Ros when she believes her to be his lover, telling Tywin about Shae, and ensuring Tyrion's death sentence as she accused him of killing Joffrey. Although Cersei is never successful in killing Tyrion, her actions create a catalyst effect that result in him having to leave King's Landing.
Invading Highgarden
After Olenna Tyrell declares for Daenerys Targaryen, the Lannister army, backed by the Tarly army, sack Highgarden using the element of surprise.
Cersei was cunning with her plan to sack Highgarden, she had House Tarly, who had historically been loyal to the Tyrells, on her side and using them enacted a surprise attack on Highgarden. Although after Olenna was killed, she wished that she had died in a more painful way, Cersei was able to use the Tyrell's gold to pay off crown debts and enlist mercenary help for her war against Daenerys.
The Arrests Of Margery And Loras Tyrell
Cersei despised Margery from the start, ultimately, Margery was the reason that Cersei lost a lot of control over her sons Joffrey and Tommen.
By conspiring with Loras Tyrell's lover Olyvar, offering him protection in exchange for betraying Loras, Cersei was able to avoid marrying him and in turn get Margery Tyrell arrested for protecting her brother. Cersei was able to enact this plan without the knowledge of Olenna, who was the real power behind House Tyrell, and though she received accusations from her after the arrests were made, Cersei had already achieved her goal.
Installing House Lannister As Royalty
Cersei listened to her father's counsel closely for her entire life, and one of the primary ways she resembled him was their shared ambiton and power-seeking.
When all of her children are dead, Cersei maintains control of the Seven Kingdoms, and proclaims herself ruler by conquest. Although it was not in Cersei's plans for her children to die, keeping her grip on the realm was cunning and sneaky, she began to make friends in high places and carry out advice her father had given previous rulers.
The Death Of Robert Baratheon
During her time as Robert's wife and queen, Cersei suffered embarrassment at his hand. He made it quite plain that he did not hold his wife in high regard, something that did not sit well with Cersei.
Cersei orchestrated the plan that would result in the death of Robert after a hunting incident, ordering her cousin and lover Lancel to give Robert more and more wine so that his senses were not alert whilst he is hunting. After sustaining injuries from a boar, Robert later dies and Cersei's son is proclaimed king. Cersei manages to keep her plans quiet and remains blameless for Robert's death as her plan appears as a terrible accident not involving her at all.
Blowing Up The Great Sept Of Baelor
Growing contempt for the faith and the Tyrell family caused Cersei to hatch a plan to blow up the Great Sept during what was meant to be her and Loras Tyrell's trial.
One of Cersei's greatest and sneakiest plans, blowing up the Great Sept of Baelor removed the Tyrells and the faith from power. Using wildfire that the Mad King had kept underneath the Great Sept, Cersei was able to burn her enemies with little chance of their escape, as well as instill fear in Margery before her death as she realized something was wrong when Cersei didn't turn up for her own trial.
Plotting With Euron Greyjoy
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Euron Greyjoy, although a distasteful man, was in possession of the Iron Fleet. Cersei understood Euron was necessary for her to stay on the throne and protect herself.
Although by appearances, Euron had abandoned Cersei to protect himself against the Army of the Dead, this was merely a plot to make it look like Cersei no longer had his support and was agreeing to a ceasefire whilst the Army of the Dead were dealt with. In actuality, Euron was going to enlist the help of the Golden Company to deal with whatever was left of the Dothraki, Unsullied and Northern Armies after they fought the White Walkers. This cunning plan gave Cersei the upper hand and kept her armies strong for when Daenerys attacked.
Source:gamerant.com