The mountain had an abudant spiritual Qi.
On the top of the mountain, most important places were Kunwu Hall, which was the heart of the mountain, the Spirit Treasure Pavilion and the Devil Suppressing Pagoda.
After the invasion of forces from the Elder Devil Realm, perfect devil Qi was present is in Mortal Realm. It could corrupt not only the spirit veins of the mortal world, but it could even spread corruption through the air, making common humans and low-grade cultivators gradually turn into devils. Given enough time, the Mortal Realm would become a copy of the Elder Devil Realm.
The Three Kunwu Masters sealed a majority of the it in the areas with the most abundant spiritual Qi, like Endless Sea or Kunwu Mountain, where it was sealed on the 9th floor of the Devil Suppressing Pagoda. They used the pressure of these sites to suppress the corruptible Qi.
On the 8th floor, Sacred Ancestor Yuan Cha's soul fragment, who fused with Ling Long was sealed in the Nine-truths Devil Suppression Formation and with 2 Divine Spirit Treasures, the Eight Spirit Ruler and the Black Wind Flag. Due to Ling Long's status of the concubine of one of the Spirit Realm's Demon Kings, the Divine Wolf Tian Kui, the Three Kunwu Masters didn't dare to harm her and only sealed the whole mountain. 
Furthermore, the Three Kunwu Masters also sealed 4 demons, the Silver-winged Nightfiend, Lion Hawk, grade 10 Vast Cliff Tortoise and a grade 10 Treant Lord, with Spirit Prison Formations in rarely-known places in the Great Jin. Their formations were designed to teleport them to the Kunwu Mountain to defend it on their behalf, when someone would try breaking the seal on it. Their Soul Binding Tiles were placed in Kunwu Hall.