Don't blame Turin, he was cursed, and his curse only affected Gondolin when Hurin was released by Morgoth. At Hurin's release he went to the encirling mountains and accidentally revealed the area where Gondolin lay. Then Morgoth was able to capture Maeglin, who was easily turned because of his jealousy of Tuor.
Also, Turgon should have listened to Ulmo!
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