The formation was composed of two different layers, like Yin and Yang, as the outer layer of the defensive barrier was also the main one.
What surprised him though wasn't the defensive capabilities of the barrier but its absorbing ability, which was also the main sustaining energy source of the formation.
In other words, the barrier was self-sustaining in case of attack, and if the first attack wasn't powerful enough to break the barrier in one hit, then it would be like feeding a tiger.
The more one attacked the barrier the more energy it would absorb, and the stronger it would become. Like this, it would slowly become impossible to break the barrier without cracking it.
The inner part of the barrier was only a complementing one, as its purpose was only an activating and helping one, as the activation energy for the barrier was extracted from the 'Eternal Flame'.