This series proves Aldiss' ability at world building and that he's not just good at writing short stories and novella's. Throughout this series (and even this book) characters come and go but the real story is that of the Planet Helliconia itself as the annual cycle of life is followed through from Spring to Winter. That's a larger prospect than it sounds given that one Helliconian year is equivillent to 2500 Earth years.
Human civillization rises and falls in the space of a Helliconian year when the monsterous Phagors become the dominant species during the winter. Observers from Earth watch silently from above in an orbital space station, forbidden from interfering for their own safety because of their vulnerability to the viruses that they have no immunity to. Slowly thoughtout the series the way the life cycles of the variety of life forms on the planet interact in a symbiotic way to assure their mutual survival is revealed. Nature imposes a harsh but necessary routine upon the inhabitants of Helliconia.
This really is a masterpiece but it is slow going and one must be prepared to realise that the characters are not the central focus of the books.