The war for the Iron Throne has begun and Tyrion has a solid plan of attack. While the Martell and Tyrell forces surround King's Landing, the Unsullied will lay siege to Casterly Rock. But where is Casterly Rock on Game of Thrones? It's on the Western coast of Westeros and is the ancestral stronghold of House Lannister.
Casterly Rock sits on a major goldmine, which provides House Lannister with their wealth. The Lannisters ancestor, Lann the Clever, hoodwinked the castle from House Casterly, who previously held the castle, thousands of years ago. Since then it's remained in the family and has been a sore spot for Tyrion in the past.
Back in Season 3, Tyrion asked his father, Tywin, to make him the heir to Casterly Rock since Jaime was the Kingsguard but Tywin refused because Tyrion is a dwarf. Though sending the Unsullied to lay siege to Casterly Rock was probably a strategic move, it's hard to believe this isn't personal for Tyrion. He's always been seen as less than and not a real Lannister. Now he's Daenerys' hand and if the Unsullied are successful, Tyrion can finally have Casterly Rock for himself; what he's always wanted.
However, this battle will get ugly. No doubt Jaime will ride in for the battle himself (he hasn't been doing much this season, anyway) and though Tyrion probably won't be there, Jaime should notice that it is his brother behind the planned attack.
The battle for Casterly Rock will also be a very difficult one for the Unsullied. The stronghold has never been taken since it was in Lannister hands and for good reason. The castle literally sits atop a rock, making it difficult to get to but easy for those in the castle to attack those below with arrows. Additionally, rumor has it the Rock is so strong it could even withstand dragonflame, though this hasn't been confirmed.
Even if the Unsullied win, it'll be a hard fought victory that will certainly come with casualties. After Grey Worm and Missandei had their very romantic good bye, fans can't help but worry Grey Worm won't survive this battle. It would be so like GOT to finally give viewers a Grey Worm and Missandei love scene, only for Grey Work to end up dead in the next episode.
Hopefully, it doesn't come to that but I'll be praying to the seven gods and the Lord of Light, just in case.