Here is the full list of all 41 Castle on the Coast achievements.
Unlock the jetpack
Survive an alternate dimension
Obtain the ocean's keystone
Taste defeat for the first time.
Unlock your first costume
Arrive at the Castle on the Coast
Discover the secret ocean
Play the world's largest musical instrument
Defeate Aleandra and obtain the forest keystone
Defeate Vendrick and obtain the rainy cathedral keystone
Obtain the stunt park's keystone
Beat the game!
Unlock Aleandra's costume
Unlock Vendrick's costume
Unlock the mecha costume
Unlock the secret costume
Unlock all costumes
Collect every spiky star
Collect every orb star
Collect every moon star
Collect every star
Accumulate 1,000 unspent petals
Spin a character 10 times
Bounce off a character 10 times
Spin a character 30 times
Bounce off a character 30 times
Cheerfully wave 30 times
DETONATE your save file.
Turn on co-op mode
Take down 15 suites of armor
Defeat 20 of your bretheren in wood
Make friends with a shark, awwwww...
Find a hidden ball
Reach the highest point of the castle. Wow!
Escape the confines of the playable castle area
Customize your appearance while wearing the kid costume
Drive a car into the library
Drive a submarine into the library?
Reach the peak of one of the large, distant mountains on the mainland
Reach the top of the distant, ocean tower
Activate the hidden player 2 camera mode