Sky Ruins

Sky ruins were added in the ascension update. At first, sky ruins would be completely optional. They were used as little challenges that needed 2 players. You could get abilities to make the game easier, but they weren't required.

These abilities include:

Double jump

Shadow warp

Speed boost

Crystal ball (sky ruin added later, sky ruin includes a hidden red key)

November sky ruin update
In November, BADGRAPHIX released a video talking about the scrapped sequels of castle in the sky. He stated that he put these scrapped sequels into the game as hidden sky ruins that can help you easily beat the game.

The sky ruins can be accessed with wings by going into the bonus room, glitching out, and flying all the way to the ruin.

The island sky ruin
The first secret sky ruin takes place in a scrapped sequel on an island, called temple in the sea. This island consists of all 4 keys needed for every monument, as well as every ability.

The obstacles you have to go through are not easy, however. The obstacles consist of black platforms, with spikes. The spikes' hitboxes are very large however, proposing a small challenge.

The sky ruin can be located at the blue key. You need a friend to activate the bride activator on a window below in order to access the sky ruin.

The 10 player sky ruin
The second sky ruin takes place in a forest from another scrapped sequel. The sky ruin contains everything from the previous island.

In order to access this ruin, you have to go inside the vent in the Sunroof Cave exit. 10 players must be in the server to access the gate.