I was eager to watch this as I've always been a castle and history enthusiast. And it has introduced me to some interesting castles that I knew little about. But frankly I was left quite disappointed by most of the episodes.
The main problem with the series is the presenter. Dan Jones is probably a perfectly good historian, but he is absolutely laughable as a tv presenter. His presentation style is so awkward and uncharismatic that you often wonder if you're watching an episode of Alan Partridge. It's genuinely quite cringe-inducing to watch him saunter about in this leather jacket he never seems to take off, standing in awkwardly staged poses with endless closeups of his gormless face staring "dramatically" at walls while he tries very hard (and again, awkwardly fails) to deliver his script in an engaging fashion. Needless to say, whatever the quality of the documentary content, this constant distraction subtracts quite a lot.
And sadly, the documenary side of things is quite lacking as well. For a show called Secrets of British Castles, the episodes often say remarkably little about the actual castles themselves. Instead what you mostly get is a very surface-level introduction to a few disconnected kings or nobles that lived in them and a few key events in their history. Nothing wrong with that per se but it just doesn't go far enough to get a real sense of the places.