In ancient times, cities were built with protective walls. If there was only one gate, then everyone would have to go through that gate to go to any household in the city. Similarly, if we cannot break through the barrier of renunciation and bodhicitta, we will not have access to any genuine, profound practices like the Great Perfection, Mahamudra, Kalachakra and others. Once we pass, the doors to the various systems of practice will all open and we can choose at will to practice Pure Land, Zen, Madhyamaka, Mahamudra, or the Great Perfection.

~ Depicted from THE RIGHT VIEW - The Three Supreme Methods—the ultimate methods of cultivating virtue and training the mind