Honored Questioner
You ask about the Treasure Tower and about Abutsu-bō. I will explain it as I understand it from the writings myself.
The Treasure Tower comes from chapter eleven of the Lotus Sutra. There, an enormous tower rises out of the earth, adorned with seven treasures, and remains suspended in the air. Inside sits Many Treasures Buddha, bearing witness that the Lotus Sutra is true. This may sound like a story far removed from your own life. But Nichiren Daishonin explains that this tower is nothing outside of you. It is the direct form of your own Buddha nature, the seed of enlightenment that lives in every ordinary person.
Think of a closed door in a dark room. As long as the door stays shut, you see nothing of what lies behind it. That closed door is your own confusion and doubt, what the writings call mumyō, fundamental darkness. When you recite Namu-myoho-renge-kyo with faith, that door opens. What becomes visible is not something new arriving from outside. It is what was already present in you the whole time.
Nichiren Daishonin wrote this to Abutsu-bō, a man of seventy living in exile on the island of Sado. Abutsu-bō had first followed another school, the Nembutsu. When he became convinced by the Daishonin's teaching, he and his wife Sennichi-ni secretly brought food and supplies to the Daishonin at night, despite a ban from the local authorities. He risked his own life for this devotion.
To this man, the Daishonin writes the following, and I give you the words exactly as they stand:
"Abutsu-bō is himself the Treasure Tower, and the Treasure Tower is Abutsu-bō himself."
And further, in the same spirit:
"In the Latter Day of the Law, there is no other Treasure Tower than the figures of the men and women who embrace the Lotus Sutra. It follows that, therefore, whether eminent or humble, high or low, those who chant Namu-myoho-renge-kyo are themselves the Treasure Tower, and they are also Many Treasures Buddha himself."
This is not a nice phrase meant to comfort. It is a direct statement about what an ordinary person is. Abutsu-bō was no priest, no scholar, no man of standing in the eyes of the world. He was old, exiled, and carried food through the cold at night. That exact body, that ordinary, mortal body, is what the Daishonin calls the Treasure Tower.
This connects to what is called sokushin-jōbutsu, attaining Buddhahood in this very body. Before the Lotus Sutra, other schools taught that a person first had to cast off earthly desires, over many lifetimes, before reaching enlightenment. The Lotus Sutra says something different. Your body as it is now, with all its ordinariness and flaws, is already the form in which the Law shows itself. The writings therefore call the body "the palace of the ninth consciousness, the unchanging reality that governs all functions of life."
On the Gohonzon, the central inscription Namu-myoho-renge-kyo-Nichiren is that same Treasure Tower, now made visible as an object of faith. When you sit before it and recite, that is not an act directed toward something outside you. It is the opening of the door that you yourself are.
深く感謝いたします
Namu Myoho Renge Kyo