The story is considered the weakest of the three, but not by a large margin. It has its own merits and is enjoyable.
The reviewer from Top Level Canon finds The Last Castle to be the weakest of the three stories they've read by Jack Vance, but not by a significant margin. They appreciate the story's unique qualities and find it enjoyable overall. The narrative is engaging, and the characters are well-developed, making it a solid read despite its minor flaws. The reviewer suggests that while it may not be the best of Vance's works, it is still worth reading for fans of the author.
Quick quotes
That said, I do think that this story is the weakest of the three even on its own merits, just not by as big a margin as you'd probably think.
The narrative is engaging, and the characters are well-developed.
It is still worth reading for fans of the author.