Keyholes

There are a total of 8 keyholes found throughout the game. Finding them all unlocks an achievement called The Truth.

Denial

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Keyhole 1
A Queen and King celebrate the birth of their son, wrapped in a red blanket. The Queen has red flowers in her hair, and the King, overcome with joy, plays a lullaby for his child on a conch shell.

Keyhole 2
The King kneels before his Queen, both of them somber. The flowers in the Queen's hair are wilting, and the entire image is dark. In the background, the Prince plays with his toys, unaware of his mother's illness.

Keyhole 3
The Prince stands before his mother's grave, her circlet in his hands. A cold wind blows through the cemetery as the King weeps in the foreground.

Keyhole 4
The King watches, forlorn, as his son sets on a boat to sail away from the land, a lighthouse is seen in the backdrop. The image is leeched of color, and a storm is rolling in over the land as the Prince doesn't look back.

Keyhole 5
The King, all alone, is overcome by his grief. Dark tendrils spread out from his form and darkness overtakes the frame.

Keyhole 6
A dark fog rolls over the land from the King's influence. Previously calm waters become violent and dangerous, and previously elegant structures fracture and crumble in the wake of the King's grief.

Keyhole 7
The Prince returns to a decrepit kingdom. Where the King had been waiting for him before, there are now only empty, crumbling buildings. The lighthouse is now the keyhole tower.

Keyhole 8
The King and the Prince reach to each other from an uncrossable divide. The King, on the top, reaches down to his son from a lighthouse framed by the moon in the night sky, while the Prince reaches up to the King from the keyhole tower, framed by the sun.

Speculation
The story behind the keyholes is strongly implied to be an allegory for Enu and Manu disguised as a fairytale. This is supported by the throne at the beginning of each stage slowly transforming from the King and becoming more and more similar to Manu, as well as the red cape that Enu and the Prince share.

The Prince sailing away from his kingdom in Keyhole 4 could be a metaphor for Enu's own death.

The imagery in Keyhole 8 in itself is a spoiler for the true nature of the game. The King -- Manu -- reaches down towards his son, who reaches back towards him from the depths of the ocean. Similarly, the King reaches down from the night sky, which is where Enu ascends to at the end of Depression and the sky we find Manu under at the beginning of Acceptance.

Trivia

 * The circlet that the Prince wears previously belonged to his mother, the Queen, which he kept when she died.
 * Underneath the red tree in Denial can be found a marker surrounded by red flowers similar to the ones in the Queen's hair. Finding this place earns you the achievement Funeral flowers. These flowers appear to be poppies, which are associated with remembrance and death.
 * The petals that blow in the wind from the Queen's grave look incredibly similar in appearance to the leaves that Nana spawns when appearing/disappearing or when using her magic.
 * It could be assumed that the story behind the Keyholes (or one similar) could be a fairytale that Manu read to Enu, from the storybook-like illustrations the Keyholes are drawn as, and the influence they have on how Enu perceives the island. This could also be suggested by the book that can be found next to Enu's bed in Acceptance.