Takeaway: The final story is strangely satisfying.
The High King is one of the best known on Young Adult fantasy world before Harry Potter. It won the Newberry Award in 1969. This was about the same time that the Lord of the Rings trilogy really took off in the US (the last of the Lord of the Rings books was published in 1956).
The High King is the most like the Lord of the Rings trilogy. It has the most action; it is the most focused on large-scale battles. It is the most exciting and up until this reading it was my favorite of the series. It is still a very good book, but Taran Wander is more interesting as a character study. I like the action books, but they are what is driving my reading. I first read the series first in the mid-1980s and was in its target audience.
The High King opens with Taran still returning from his wanderings. Eilonwy has returned her princess training and Taran is ready to ask her to marry him. He actually starts to propose when Flewddur Fflam shows up with an unconscious Prince Gwydion.
That starts the action as Arawn has left his stronghold for a final press against High King Math. Taran becomes a war leader of the freehold area that was the setting for Taran Wanderer.
Throughout the book, Taran feels the pressure of leadership and has to learn more and more to stand on his own as his friends and mentors leave Taran for one reason or another. As I said with Taran Wanderer. Taran is now grown and instead of learning what it means to be a man, Taran learns what it means to be a real leader of men.