Fastest way to get gold in Castle Crashers!
How To Make Money
As the Industrialist or Fencer, make your way to the Wizard Castle Interior.
Use continous splash magic attack to make short work of the Painter.
Collect loot. He should drop 300-400 gold per defeat, and can be defeated in less than 30 seconds.
This method can be used on any boss that does not get knocked down, though I have found the Painter to be the quickest, with most gold given per defeat.