There was a large variety of array techniques. There were some that focused on bewitching the senses and were capable of imprisoning those caught in their threshold to their death. There were others that were killing arrays, and whoever entered would encounter an endless barrage of murderous traps. There were also some arrays that were mainly meant for defense and could be as sturdy as an iron barrel.
Mountain Protection Great Arrays typically focused on defense, and the one laid out for the Imperial Palace of Fire Country was as such. Though it was a Sixth Tier array technique, it would be very difficult for even a Spiritual Infant Tier cultivator to force his way in. That was because once an opening was created in the defenses, it would very quickly close up. Thus, only if one was powerful enough to create a gigantic opening could he pass through calmly.
That would at least have to be one in the middle period of the Spiritual Infant Tier.