Programmer Tom Fulp, founder of flash site Newgrounds, also founded a video game development company called Behemoth to release titles made with the intention of bringing what was considered "old-school" styles of gaming into the mainstream, which started with run-and-gun style gameplay with Alien Hominid in 2004, and then eventually beat-em-up style gameplay with Castle Crashers in 2008.
With animation drawn in artist Dan Paladin's signature flash style, and a soundtrack supplied solely by various music-makers on Newgrounds, Castle Crashers feels not too different to play than a flashgame you could find on Fulp's site; and for the cult fans of Fulp that's a good thing, since that's the style that they probably love about Behemoth; but considering this, remember you can play any Newgrounds game for free, and yet you have to pay a hefty $15 to play Castle Crashers.
If you like the gameplay, you'll probably say it's worth the price, since there is a steady stream in how Castle Crashers showcases its variety of enemies, bosses, level settings, and humor throughout each of the levels in the campaign to keep you addicted until you've played it all the way through. In short, Castle Crashers is functional; with the exception of a few peeves:
The 2D animation becomes a flaw considering the traditional usage of 3D level depth in combat-- enemies will appear to be right in front of you, so you attack, but then you would miss since the game registers the position of the enemy and your character to be beside your swing rather than in front of it. Bigger characters that are prevalent in the game (like the many huge bosses) will also frequently hide your character mid-fight as they walk around the level plane in front of you (and sometimes this even happens with enemies the same size as you if they are close enough). So it's likely a pretty big portion of your time playing Castle Crashers will be spent wondering where the hell you are.
Outside of the campaign there are several trivial minigames only dedicated players and fans of Fulp will acclaim or endorse.
As much as I want to like a game with such stylistic art, playful tone, and entertaining black humor, I still find the gameplay itself rather tedious. None of the loud and loyal Castle Crashers fans would truly like the game as much as they do if it weren't Tom Fulp or drawn in a less flashy and appealing art style. Replace the backgrounds and characters with some simpler animation and suddenly you have yet another generic beat-em-up game with a sense of humor that's only half-redemptive.