Gondolin was founded by Turgon in the first age of Tolkien's middle earth. Gondolin was built in a secret valley. And Ulmo which is a Valar has shown this hidden place. The secret city of Gondolin has evolved over the years and has become a very spectacular place. Morgoth searched for the place of Gondolin for 400 years. And in the end, Morgoth, thanks to Maeglin, found Gondolin. Morgoth attacked Gondolin with a huge army. And so the secret city of Gondolin was destroyed by Morgoth's army.