Crown of the Hidden Kingdom

From Tolkien Gateway

The Crown of the Hidden Kingdom was the king-helm of Turgon, king of Gondolin.

An image of the Crown was placed above the midmost pillar of the last gate, the Gate of Steel, and it was set about with diamonds. The gate would open inwards on either side of the pillar with the Crown above it.[1]

Other versions of the legendarium

In the Lost Tale of The Fall of Gondolin, Turgon is seen wearing a coronet with garnets upon his head. And later during the assault on the city, the king had thrown his crown at the roots of Glingol. Galdor of the Tree had picked it up, with the intent of returning it to Turgon, yet the king had refused. With the crown in Galdor's possession, it would escape the destruction of the city.[2]