The Martells of old used a spear as their emblem, while Nymeria and her Rhoynar used the sun as theirs. When Nymeria wed Lord Mors Martell, the symbols were combined into a gold spear piercing a red sun on an orange field. Their words are Unbowed, Unbent, Unbroken.
What she means: House Tyrell is the second most powerful house in all of Westeros, from the amount of gold they have, to their massive armies which are easily supported by their gold. They still have a number of powerful members of their family even with Margery and Loras dead. In addition to that they have spies, bards, etc, all over the seven kingdoms just like everyone else. Casterly Rock is incredibly far away from High Garden and there’s no way in hell that pretty much the entire Lannister army would march all the way to the Tyrell’s capital in High Garden with literally nobody noticing; and even if they got there the Tyrell’s army isn’t exactly non-existant. To fight them the Lannister’s would’ve easily sacrificed such a large amount of their forces making their chances against Daenerys almost negligible, all the while making Kings Landing and other Lannister controlled territory so incredibly weak that Daenerys could’ve waltzed into Kings Landing without shedding a single civilian blood drop. On top of that Olenna is one of the most incredibly intelligent and powerful characters on the entire show and wouldn’t have been eliminated from this unrealistic attack, and she also has more than a single castle she could’ve retreated to. Also Euron’s fleet just appeared out of nowhere with zero warning as well? Just like in the fight with the sand snakes? Where he defeated three of the show’s most powerful women at once? All of this has but one explanation: Sloppy storytelling
The real tragedy of the book to show conversion is that the sand snakes actually wanted to crown Myrcella, not kill her.
See, Dorne is the only kingdom that treats women as if they’re equals. Women inherit lands and titles there. In fact, Doran actually has a daughter, Arienne, who leads the sand snakes in this plot. Oh, and she’s the crown princess of Dorne. Trystane is such a minor character.
They want to crown Myrcella, because according to Dornish law, she should have rightfully become the next ruler of Westeros instead of Tommen. Their revenge on Kings Landing isn’t to kill another Lannister, it’s literally to create a civil war against patriarchy. To make the Lannisters chose a side.
So by erasing Arienne, and making the sand snakes entire plot revolve around wanting to kill an innocent girl, the show really destroyed one of the best plot lines and kingdoms in the books.