Here is the full list of all 16 Secrets of Arcadia achievements.
Find and open all standard treasure chests
Unlock every bonus chest
Defeat King Lameses
Purchase every skill that is available to your character
Complete the game with the Ranger
Complete the game with the Warrior
Complete the game with the Mage
Complete the game with the Paladin
Find and defeat both of the Golden Knights
Defeat 250 enemies in a single playthrough
Find the portaits of the dungeon builders
Start a co-op game
Complete the game in co-op mode
Complete the game without dying
Collect 7,777 gems
Complete the game with every character