The Moon Hua Sect probably placed the "Twelve Golden Lotus Art" behind a wooden door because they were afraid that the more talented Inner Sect Disciples might accidentally cultivate it.
After all, typically, disciples who have just entered the Inner Sect are unlikely to choose such a difficult to cultivate martial art that requires very high standards and is extremely dangerous.
Advancing to the Transcendent Realm is not easy. If one fails, all previous efforts are wasted, and the price to pay is a bit too great.
Upon deep contemplation, Ling Xiao discovered that the "Twelve Golden Lotus Art" has two characteristics.
The first one is similar to most Internal Cultivation Mind Techniques. Each additional petal of the lotus increases the realm by one layer, and also the total amount and quality of the Martial Artist's True Essence and cultivation level, up to twelve layers, which is three more layers than most Internal Cultivation Mind Techniques;