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