Is Castle Crashers of of those games where you need friends to play with for it to be fun?

yea . playing friends will be fun .

I think it'd just be harder by yourself.
And more fun being able to socialize with your friends.

I've played it alone and it was still fun. More fun with friends though.

I have played more alone than with players online on the XBLA version, it's still a decent game even when playing alone.

Playing with friends is always fun , unless they are jerks then no :/

Mostly yea but as every game it's more fun with friends. It's still fun when you play alone tho.