[The Forbidden Tower] continues from The Spell Sword; the alien Catmen have been vanquished, though at heavy cost. Damon Ridenow and Andrew Carr marry the twin sisters Callista and Ellemir of the house of Alton. Both men are displaced persons. Andrew has given up all his ties to Earth and is feeling his way in a new culture. Damon has been denied his vocation as a Keeper, one of the highly trained telepaths who work in the Towers. The ancient science of the Comyn lords of Darkover centres rigidly upon the training and discipline received in these towers and is hedged with prohibitions.

The force of these taboos falls most heavily on Callista, who renounces her vows as a Keeper, trained in the Tower of Arilinn, to marry Andrew, the man who saved her psychically and physically from the aliens…. Keepers, through their psychic power and their link with a sentient matrix jewel, are literally untouchable: the attempted violation of a Keeper, for instance, would result in the death of the rapist. This power is exercised unconsciously, like a reflex; Callista, who loves Andrew deeply and whose defences were lowered somewhat during her rescue, must wait until she loosens up again.

This appears at first a rather titillating problem, a private difficulty impinging on the bustling public life of the great Alton house at Armida. But those readers who anticipate a cheerful defloration about the middle of the book have underestimated Marion Zimmer Bradley. Callista's cruelly imposed frigidity is at the very centre of the book and it is examined with increasing depth and widening implications until the final pages….

The way in which the threads are gathered up: the need for a wider use of telepathic healing, the superstitious narrowness of the taboos, the painful and unnecessary discipline imposed on the immature female adepts …, even the dynastic implications of Callista's marriage, this interweaving is skilfully done.

The mechanism of Darkover's psychic world with its paraphernalia of screens, monitoring, matrices and trips to the astral plane or overworld, is described with firm authority. The background of Darkover is beloved and familiar territory for the author. We have the feeling that she no longer invents Darkover, she simply goes there. The culture is nicely balanced between a harsh environment, a feudal society complicated by the presence of telepaths and a high degree of sexual liberty and closeness. (p. 106)

It is fair to ask just how well the characterisation stands up in a book where the four main characters Damon and Ellemir and Andrew and Callista end up closer than the average husband and wife. Are the twin sisters Ellemir and Callista simply another example of fairy-tale splitting of the Frodo/Sam or closer still, the Lethonee/Sorayina type? The verdict is "not proven"; there is more to both girls than a simple warm/cold duality. Damon is the best developed character and Andrew, we notice, becomes more sympathetic as he is drawn into the culture of Darkover. The Keeper, Leonie, hovering on the verge of myth, and the old lord, Dom Esteban, wholly human, are well-drawn supporting characters.

All this is done in a loping, down-to-earth style; we have a sense not so much of padding but of purposeful backing and filling. The writing is not pretty but it is not inflated; the author rises to the occasion many times. The episodes in the over-world, where Damon has built a small shelter and must later expand it into the Forbidden Tower of an independent Keeper, are well done. The adventure in time is appealing and perhaps there exists already in the mind of the author or in an earlier book, the same scene from the point of view of Damon's ancestor, Varzil, confronted by a descendant from the future. It is a measure of the seriousness of the work that this episode stands out almost as light relief; the total impression of the book is one of cumulative psychic power…. (pp. 106-07)

The personality of the author, tough-minded, practical, spiritual, hums in the background of this novel like a matrix jewel. Marion Zimmer Bradley writes with a moral purpose of Victorian intensity, but it is liberal and liberating. (p. 107)