Here is the full list of all 12 Castle Crashers achievements. It takes around 15-20 hours to unlock all of the achievements on Windows.
Find and dig up ten buried items.
Defeat any end boss without using any magic.
Using a catapult, fling a knight with maximum power.
Complete the game using any character.
Collect all 4 princesses' kisses in a multiplayer game.
Defeat any boss by playing one of his own minions.
Navigate the Abandoned Mill without hitting any obstacles.
Complete the Home Castle through Barbarian Boss areas without attacking any foes.
Collect all the animal orbs.
Win 40 Arena Online Matches.
Survive until 2:30 in a Back Off Barbarian match.
In a 4 player game, resuscitate each of your fallen comrades at least once.