The characters investigate plenty of heinous crimes, but the show has a lighthearted tone overall. Castle's unusual background gives him a unique perspective that often helps them figure out who did it. He's also a doting single father to a teenage girl, and he's a bit out of his comfort zone now that she's starting to date.
Little on-screen violence, but most of the crimes on the show involve violence -- including murder -- and the crime scenes sometimes feature brief shots of corpses.
Castle is a charming rake who constantly flirts with his female partner and the women they meet during their investigations. They also sometimes speculate on the sexual habits of their victims and suspects when it's relevant to the crimes they're investigating.
Drinking, Drugs & Smoking
Characters sometimes discuss drugs and alcohol as possible factors in the crimes they're investigating. Some social drinking.