In The Lord of the Rings, the great Elven city of Gondolin was one of Tolkien’s most spectacular additions to the mythos of Middle-earth. It was known for many things during its reign in the First Age of Middle-earth. First and foremost, it was known for the secrecy of its location. Only a few outside the city itself knew of its location or how even to find it, which is why Morgoth both feared the Elves of Gondolin and sought to destroy it.
Source：https://www.cbr.com/lotr-gates-of-gondolin-explained/
