It’s based on Game of Thrones, The Wall is 700 feet high and protects the Seven Kingdoms from the Others. Castle Black is the headquarters of the The Night’s Watch, the group charged with patrolling the wall.
I’ve never watched the show, but the books are great and I’ve been reading through them for the first time recently. Just know before you get into it that the author very notoriously hasn’t finished the series in like… 20 years, haha. There are 2 more planned books but the last came out in 2011.