The icon for Vaisravana shows a heavenly king in Genghis Khan-type armor. He holds the banner of authority in one hand and a stupa, a reliquary tower, in the other. But where are the weapons which can forcibly subdue the non-believers and protect the Dharma? Can a stupa protect Buddhism?
Yes, it can. This is the iron tower into which Nagarjuna entered to receive the Vajrayana. The stupa represents samadhi, and Vaisravana ("extensive hearing," Bishamon-ten, Tamon-ten) offers the protection which comes from entering samadhi and receiving the Dharma. The icon shows a brilliantly shining sun inside the stupa. This is the Buddha's relics; this is the Dharma he left for us.
Weapons which harm people will not protect the Dharma. The warrior skills of martial artists will not protect the Dharma. In the Avatamsaka Sutra, Sudhana nears the end of his journey to enlightenment and comes to a great tower containing the adornments of Vairocana. He prostrates himself in front of the door, and then circles the tower respectfully hundreds of thousands of times. From a distance, Maitreya ("The Loving One") comes and opens the door for Sudhana to enter, and "Sudhana, in greatest wonder, goes into the tower. As soon as he entered, the door shut." The following 10 pages describe that samadhi.
Right from the very beginning of Shakyamuni's preaching, that is, the Eight-fold Noble Path, samadhi was the culmination of that Path. The only protection for the Dharma is for people to enter into samadhi and realize it for themselves.