Jung, Memories, Dreams, Reflections, p223:

Gradually, through my scientific work, I was able to put my fantasies and the contents of the unconscious on a solid footing. Words and paper, however, did not seem real enough to me; something more was needed. I had to achieve a kind of representation in stone of my innermost thoughts and of the knowledge I had acquired. Or, to put it another way, I had to make a confession of faith in stone. That was the beginning of the “Tower,” the house which I built for myself at Bollingen.