Castle is comfort-food TV—a murder mystery you can solve from the couch with a bowl of popcorn. It's not prestige television, but it doesn't pretend to be. The chemistry between Fillion and Katic carries eight seasons, and the show knows how to balance humor with crime without tipping into either pure comedy or grim procedural territory.
That said, it's firmly in the 13+ camp. Every episode delivers a fresh corpse, the sexual banter is constant (if not explicit), and there's enough profanity and mature themes that you wouldn't want a 10-year-old wandering into the room mid-episode. For families with older teens who want a co-watch that isn't too intense or too juvenile, Castle hits a sweet spot.
It's dated now (2009-2016 vibes are strong), but not so much that it feels unwatchable. Think of it as the TV equivalent of a beach read mystery novel—entertaining, low-stakes, and you'll forget most of it by next week. Which is sometimes exactly what you need.