"Game of Thrones" Season 7 would see Daenerys arriving in Westeros and by the looks of the trailer, it seems like her forces would succeed in capturing Casterly Rock and now a Redditor has come up with a strong theory backing the assumption. It says that Tyrion Lannister's knowledge of the sewers and drains of the castle (something he referred in Season 2) could help Daenerys in seizing the impenetrable castle.
"Game of Thrones" Season 7 will finally give Daenerys a taste of power struggle in her homeland and it is said that Tyrion Lannister would play a pivotal role in helping her seize Casterly Rock. According to Time, a Reddit user has come up with a theory that says that the strong castle might come under Daenerys's control with Tyrion's help as he knows the ins and outs of its drainage system. If fans remember, Season 2 had seen Tyrion revealing this detail to Varys.
When Stannis' army was heading to the King's Landing, Tyrion was seen telling Lord Varys that his father Tywin Lannister had made him in charge of the sewers and drains of Casterly Rock when he was an adult. It acquainted him with the secret routes of the castle and if the Reddit user is to be believed, he could use the same in helping Daenerys seize the impenetrable fort.
Also, Tyrion understands the minds and hearts of his brother Jamie and sister Cersei. He knows how they operate and thus, he could use these resources to help Daenerys' cause in "Game of Thrones" Season 7. This would also make sense as Casterly Rock is a very strong and well-guarded fort. Those who have read "A Song of Ice and Fire" would know that the book says that the castle is impossible to be seized. "Game of Thrones" Season 7 premieres on July 16.