I saw someone suggest that this castle would be used for Highgarden. They were mistaken.
At the WotW article is more info on what interiors they’ll be using and what sort of scenes they’ll do at Horn Hill. We’ve already heard they’re looking to cast a baby at this location, which means the Tarlys will meet Gilly and Baby Sam.
What I find most surprising is that they’ll be using that red bedroom as an interior. That’s the sort of bedroom I’d expect to see in a Dornish castle. Perhaps the Tarlys have some Dornish ladies in their family line.
Sam’s mother, in the books, is Melessa Florent; she’s from the same House as Stannis Baratheon’s wife. We probably won’t get that detail on the show. Sam and Dickon have three sisters; one is named Talla, and the other two names are apparently undisclosed. Everyone in the comments at WotW is talking about what this filming news says about the likelihood of Ian McShane playing Randyll Tarly. I’m not sure the location tells us anything about Randyll’s casting. I do sort of feel like, having announced Dickon’s casting, now would be a good time for the good people at HBO to tell us who’s playing Randyll. If they haven’t done so already, he’s probably not an actor most of us would recognize.
Another interesting tidbit in this casting news concerns timing: the Horn Hill scenes will probably be directed by Jack Bender, which means they’ll be in episodes 5 and/or 6, which means Sam will have already completed most of his journey to Oldtown by E6, which means that Arya’s swim in the canal takes place too late in the season to be an interaction with Sam. Arya’s dip in the canal is part of E7. Which means that if Sam and Gilly visit Braavos on the way to Oldtown (and there’s no reason why they must), they’ll already be long gone before Arya takes her dip. Although, now that we’ve heard Arya is returning to the Riverlands in S6, we don’t even quite know that her swim takes place in Braavos. She could possibly be swimming in the Trident!