This was all right, but not as good as Castles. It's all serviceable, and it filled in some gaps in my knowledge of the legends of Britain before Arthur.
Castles had a neutral non-fiction editorial tone, with the main artistic expression coming from Alan Lee's pencil. The art in The High Kings is a downgrade but still serviceable fantasy illustration.
The writing is the bigger problem.
It's a mix of short non-fiction essays interspersed with longer re-tellings of pre-Arthurian legends of the Britons and Celts that inhabited "The Island of the Mighty" before the Saxons. These fictional re-tellings are based on the fragmentary remains of Celtic legend and myth, framed by small scenes of Arthur and his knights (rendered as more period-appropriate Celtic warlord and his warband, instead of the familiar but anachronistic Medieval take).
The downside is that in this format, the book very much takes on the authorial voice and style of Joy Chant which is...pretty middling and forgettable. She wrote three fantasy novels in the 70s which were fairly well regarded in their time but today she comes off as dry. I also suspect she's injecting a fair bit of 20th century mindset into this.
I assume most people come to this as "I want to know about pre-Arthurian legends" and not "I want to hear Joy Chant's take on pre-Arthurian legends". I don't think her filter is adding much value.