they want the Unsullied out in the open:
Your brother and his new queen thought you would be defending Casterly Rock.
The truth is Casterly Rock isn't worth much anymore.
Well, it is to me, but my fond childhood memories won't keep Cersei on the throne.
So you just let them take it? For now.
They won't be able to hold it.
Euron Greyjoy's navy burned their ships, we emptied the larders before we left.
Eventually, they'll be forced to abandon their position and march all the way across Westeros.
And you took your army, your real army, and went where they weren't.
As Robb Stark did to me at Whispering Wood.
There are always lessons in failures.
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Game of Thrones s07e03 Episode Script | SS