One of the best if not THE best beat em up game I have ever played. Beautiful, funny and entertaining.
You and your friends (There is local or online multiplayer) can play as one of the four knights that go around this very well designed 2D world fighting the enemies that every area has to offer (Every level has it's own: snow world has eskimos, lava world has demons) and looking for the kiss of one of the four kidnapped princesses. Every knight has it's own stats and magic, that can be upgraded by leveling him up. You will also unlock more playable characters as you play. Apart from magic, you can customize your style of play with consumables (Healing potions, bombs), weapons (That can deliver critical hits and upgrade some of your stats as long as you use them) and animals (These also help your stats and some even help you in battle); the weapons and the animals can be found in the wild or in shops, this add some replayability to the game because you might miss a secret weapon or animal or you will need to play some levels again in order to get gold and be able to buy them.
While the game might not be as challenging as let's say a Dark Souls game, you still need to be careful because if you get hit by more than one high level enemy at the same time, you can lose a lot of health or even die really quickly. Another challenge that this game offers are the bosses, there are lot of them throughout the game and they go from a huge dragon to a giant killer corn. As you can imagine each one requires some time to figure out how to kill them.
The game has some very silly humour that still manages to be funny at least for me and my friends, it's a type of family friendly humour that can make everyone laugh.
The campaign is the best and most important part of the game but there is also online mode which as of 2019 you can still find people playing (Co-op at least, versus not so much); there is also the "Back off barbarian" gamemode which has a really challenging achievement of surviving 2 minutes and 30 seconds. Another thing I need to point out is the awesome soundtrack this game has.
Overall Castle Crasher is really fun with or without friends, I recommend not only this game but also the other releases that the developers of this have such as Battleblock Theater and Pit People.