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