The very frequency and violence of temptations showed him at last the truth of what he had heard about the trials of the saints. Frequent and violent temptations were a proof that the citadel of the soul had not fallen… (Joyce, A Portrait of the Artist as a Young Man)

McLuhan was evidently struck by Joyce’s formulation here of “the citadel of the soul” since he repeatedly used variations of the phrase in the 1950s:

The Mechanical Bride, vi:
criticism is free to point to the various means employed [in art works] to get the[ir] effect, as well as to decide whether the effect was worth attempting. As such, with regard to the modern state¹, it can be a citadel of inclusive awareness amid the dim dreams of collective consciousness.

The Mechanical Bride, 87:
No longer is it possible for modern man, individually or collectively, to live in any exclusive segment of human experience or achieved social pattern. The modern mind, whether in its subconscious collective dream or in its intellectual citadel of vivid awareness, is a stage on which is contained and re-enacted the entire experience of the human race. There are no more remote and easy perspectives, either artistic or national. Everything is present in the foreground. That fact is stressed equally in current physics, jazz, newspapers, and psychoanalysis. And it is not a question of preference or taste. This flood has already immersed us.

Sight, Sound and the Fury, 1954
What we have to defend today is not the values developed in any particular culture or by any one mode of communication. Modern technology presumes to attempt a total transformation of man and his environment. This calls in turn for an inspection and defense of all human values. And so far as merely human aid goes, the citadel of this defense must be located in analytical awareness of the nature of the creative process involved in human cognition. For it is in this citadel that science and technology have already established themselves in their manipulation of the new media.

Educational Effects of Mass Media of Communication, 1956:
And no matter how many walls have fallen, the citadel of individual consciousness has not fallen nor is it likely to fall. For it is not accessible to the mass media.²

Printing and Social Change, 1959:
The literate man is one who is accustomed to the inner translation of sight into sound and of sound into sight, a complex activity for which he pays by psychic withdrawal, a weakening of sensuous life and a considerable lessening of the power of recall. But in return he obtains analytic mastery of specific areas of knowledge, and especially the power of applied science for social purposes. The increase of inner self-awareness resulting from the incessant translation of sound into sight and sight into sound also enhances his sense of individual identity and fosters that inner dialogue or conscience within, which we rightly associate with the very citadel of civilized awareness.

Characteristic for McLuhan was the attempt to bring together a way of describing art (derived especially from Pound) in terms of “individual consciousness” as the “citadel of inclusive awareness”, “vivid awareness” and “civilized awareness” — with science: “For it is in this citadel that science and technology have already established themselves in their manipulation of the new media.” Both art and science, separately, had come to develop an “analytical³ awareness of the nature of the creative process involved in human cognition”. Now was the time in which the two needed to be brought together explicitly in order to harness “the power of applied science for social purposes”.

Just this was already the message of McLuhan’s programmatic letter to Innis early in 1951:

One major discovery of the symbolists which had the greatest importance for subsequent investigation was their notion of the learning process as a labyrinth of the senses and faculties whose retracing provided the key to all arts and sciences (…) Retracing becomes in modern historical scholarship the technique of reconstruction. The technique which Edgar Poe first put to work in his detective stories. In the arts this discovery has had all those astonishing results which have seemed to separate the ordinary public from what it regards as esoteric magic. From the point of view of the artist however the business of art is no longer the communication of thoughts or feelings which are to be conceptually ordered, but a direct participation in an experience. [Similarly] the whole tendency of modern communication whether in the press, in advertising, or in the high arts is toward participation in a process, rather than apprehension of concepts. And this major revolution, intimately linked to technology, is one whose consequences have not begun to be studied although they have begun to be felt. (…) As mechanical media have popularized and enforced the presence of the arts on all people it becomes more and more necessary to make studies of the function and effect of communication on society. (…) [However] the fallacy in the Deutsch-Wiener [cybernetics] approach is its failure to understand the techniques and functions of the traditional arts as the essential type of all human communication. (…)  There is a real, living unity [of art and science] in our time, as in any other, but it lies submerged under a superficial hubbub of sensation.