What drives me nuts is that Euron started out in King's Landing when he was buttering up Cersei. From that point no matter what he does he HAS to sail past Dragon Stone. So unless everyone in Danny's administration are complete morons they'd have to know Euron left King's landing and knew which way his ships were going. And it's not like they didn't know Euron was trying to get up in Cersei's business, which means they knew he was the enemy.
It's just such a huge hole it drives me insane. And to get to Casterly Rock you gotta sail around the continent, unless it's possible to go up the river past the Twins, I don't know if that's been covered yet, but, at that point you're taking a whole fleet up a river right through the continent, there's no way you could keep that a secret. Maybe this is bothering me more than it should, but it seems like just mega sloppy writing which makes me wonder about what's going to happen in coming episodes.
It really feels like they're forcing the Lannisters to be scarier than they should be. They're broke, their only allies are Euron and the Tarleys, all of Dorn is against them, all the Northern Houses, they lost the Freys. So really all they have for an actual land army is whatever is in their own garrisons, plus whatever the Tarlys have, but remember the Tarlys were subservient to the Tyrells, so they can't have THAT many dudes. The Lannisters SHOULD get shit stomped but I think the writers are forcing them to be the big bads when it doesn't make any sense at all.
Really feels like the plotting has gone off the rails.
EDIT: And it's not like I noticed a hole no one else has, the banker guy basically told Cersei all this shit so the writers HAVE to know they're forcing shit. Maybe next week's episode will make it all clear. (I hope so, I have really enjoyed GoT thus far, it's other missteps haven't really bothered me and felt minor, not sure why this episode is bothering me so much)