The Tower of the Heart is the center of secrets of time travelers and, presumably, a place of maximum temporal power. Its location is unknown; it is reached through the end of the Blue Book.
Secret of the Heart
The ancient Greeks believed mind and memory resided in the heart, not the head. The secret was “they write with the heart, not the head”. See Secret of the Heart.
Access
The group unlocked the Planar Codes research through the revelation of the Supra-Father (Michel’s revelation). Further access requires completing the research in the Blue Book. The tower is presumably the final goal of the Templars of Time.