The Painter has a grudge against/obsession with the Orange PrincessThis would explain a great deal; Why there are pictures of her dead in its boss chamber, what the Painter's doing teaming up with the Wizard, and why the Wizard took along the Princess from the Industrial Castle instead of just leaving her there.
The Cyclops and the Groom weren't really evil.Apart from the Cyclops's magma fortress and the sinister conehead helmets the Groom and his knights wear, there really isn't much evidence for them being bad guys. The Cyclops had no idea that the bride he was purchasing from the Theives had been kidnapped. Cyclopses aren't known for being too bright; he was probably just happy that he'd found someone for his best mate to marry. For all the Groom knows, you're trying to kidnap HIS bride-to-be. When you kill him, the Cyclops finds his dead body. Does he attack you in revenge? Nope, he runs out of the church crying; he seems SCARED of you. You later find him holding a funeral for his poor deceased pal. Does this seem like remotely evil behavior? One final thing that clinches it; when you defeat the undead Groom and Cyclops near the end of the game, their souls pop out of their bodies and ascend to heaven. No other boss does this, proof that among the villains of Castle Crashers, these were just two guys who made an honest mistake.
The Orange, Red, Blue and Green Knights are the second team of knightsThe Blacksmith was the Purple Knight, as evidenced by the fact that he would appear to have a purple cross on his chest. The King was The Gold Knight, was the eldest, and eventually became The King. the Nurse in the Animal Ark was the Pink Knight, because you gotta have a token girl. and, of course, the Necromancer was The Black Knight, who was always unstable, and eventually either betrayed the other knights and joined up with The Wizard or died and then was resurrected by The Wizard and forced into working for him against his will, thereby driving the others to retire and pass on their duties of basically being super heroes onto the next group of people, and the party at the beginning of the game was the celebration of The Red, Orange, Blue, and Green Knights becoming the new protectors of the kingdom.
The Orange Princess was killed somewhere along the line.When confronting The Painter, there are assorted pictures in the background of surreal things. However, the one that seems to pop up the most sticks out; The Orange Princess with a knife in her back. Either he's obsessively drawing about doing something particularly nasty to her until you show up, or he's letting slip a plot by the Wizard in case the original plan fails. One last "Screw you" to the knights to let them know in some way they also failed.
Alternately, the Orange Princess never existed, and the implications that the "real" Orange Princess was killed are only there to add to the lie.
Another alternative, the Orange Princes wasn't killed yet, though the Evil Wizard did intend on killing her.
Yet another alternative, the Orange Princess we see throughout the game is actually the Pink Princess, we never actually see the real Orange Princess at all.
The Green Knight is a skunk.The white stripe on his helmet, his character portrait with the stink lines, his magic appears to be stink, and you never get to see what he looks like without the helmet.
The Industrialist is the result of HominidsPerhaps he started out as a simple scout drone, but decided to screw that nonsense and moved on
The Pink Knight is really Tricky the clownThe Pink Knight's projectile spell creates penguins and weird pink bear things that we see in exactly one other part of the game: the sequence after the credits with you-know-who. The Blacksmith, King, and Necromancer even establish a precedent that NPC's can be present alongside a player character iteration of themselves, so there's no problem with having both the Pink Knight and Tricky the clown in the final scene.
The princesses are somehow related to the four main knightsEach princess shares a color with one of the four main knights (orange, blue, green, and red), indicating some kind of link between the two groups. One possibility is that each princess had been promised to her corresponding knight as a bride prior to their abduction at the start of the game. Another possibility is that the king's kingdom is divided into four territories, each with a main knight and a princess.
The playable Gray Knight is a CommanderWe see most of the knights in the kingdom are gray, but it could be that the colored knights represent other territories of the kingdom which weren't close enough to the fight to help. Each Castle Crasher is then a commander of their prospective colors which is why the playable gray knight is so much stronger than the NPC grays. His is the battalion that serves as the personal guard of the king. This is taken from other WMG's. The only reason the other commanders were at the King's palace was to claim the princesses as they wives.
The ending of Battleblock Theater explains how Hatty got into the Castle Crashers universe.Upon being revived/reawakened via the hat, Hatty was transported to this medieval fantasy world. As for his motive for rescuing the Princesses? I'll leave that to you.