Here is the full list of all 26 You Have to Play This Game achievements.
Reveal all the map
Beat the game
Find a feather
Defeat Autumn
Defeat Summer
Defeat Spring
Defeat Winter
Solve the Sphinx' (real) riddles!
Obtain all Hearts for Violet and Azure
Collect all feathers.
Collect all Golden Monsters
Beat the game in newgame plus!
Rescue all princesses, free all towns, and obtain all jewels.
Obtain all conversations.
Open the gateway to Mount Celestia!
Obtain all conversations, all hearts, and all feathers. Then beat the game!
Kill a gold monster.
Collect all paintings
Complete Boss Rush in under 6 min