Game Of Thrones Season 7: 11 Major Plot Hints From The New Trailer
11. Sneaking In To Casterly Rock

HBO started teasing a battle between the Unsullied and Lannisters in the previous trailer, but we get a bit more context about how that goes down from this new footage.
Casterly Rock is considered one of the most well-defended castles in Westeros, so how exactly do the Unsullied breach its defences? In this trailer, we see Grey Worm wearing full battle armour in a boat, headed towards a cave.
We already know that Tyrion was in charge of drains and cisterns at Casterly Rock, meaning he has a working knowledge of the place. It seems he's passed that on, giving the Unsullied a secret entry point to the castle - which then allows them to let more Unsullied in from inside.
Furthermore, while the first trailer suggested Unsullied dominance, this one says it won't all be going their way, with a shot of a few of them lying dead with arrows in them.