No, not an homage to Castle Crashers: an homage by Castle Crashers. The developers have publicly stated being influenced by the classics, and it shows. Now for an attempt to cover the basic elements of design that all beat 'em ups are held to:
Thugs + Baddies
1. 1 in 6 enemies will be just like the rest, only huge. Players must hate every single one of them. They must have twice the life of a regular guy and they must do more damage and difficult-to-avoid moves. Also, these guys are the only non-boss characters allowed to grab the players.
2. At least one baddie should be doing something that looks extremely out of place just before getting into a brawl. Sitting or leaning while pretending nothing is happening is common. Also permissible is eating, drinking, smoking, or (in CC) reading a newspaper.
3. There must be at least three creative ways for baddies to enter the scene other than simply being on it when the player arrives there. Suggested ideas include jumping through windows, opening doors, dropping from ceilings, taking teleports, and arriving on moving vehicles.
4. For at least one point in the game, the player must be surrounded by a shitload of enemies. Watch the first half a minute of this video:
Level Design + Theme
1. The dramatic beginning: there is either a death or a kidnapping at the beginning and the fight is done out of a need for revenge or rescuing. In every beat'em up there must be at least one beautiful woman, and at least one is either in need of rescue or a selected character.
So, a Turtles game has no selectable woman, but you must rescue April. In a Streets of Rage game, there isn't a woman to rescue, but you can kick their asses with Blaze.
There is either a death, kidnapping, or both.
2. It's best if it shows the boss or collection of bosses take something at the beginning. It can be the girl, some public official, an object, or both. Better if the final boss is part of it.
2a. Ideally, a different version of this will be occur at some point in a later level.
3. At least one level must include traveling through air or water in an object as if you were driving a car. Better if both.
Sorry for the ugly pic, but you get the point.
4. There should be at least one level or a part of a level where fighting occurs on a high-speed object. The object doesn't arrive at its destination until you beat up all the guys who are chasing or awaiting you. It is preferable that if the player falls off, the players takes damage and is magically dropped back on the object.
5. There needs to be an elevator ride that doesn't end until you beat up all the guys who jump on to or into the elevator.
6. An obstacle course on a high speed object is optional. It must be one of the most memorable parts of the game.
7. There should be a finger, hand, or arrow that tells the player to go, even if it's not an arcade game with a time limit and doesn't need it.
7a. It is preferable to have a pointless, slow, time limit.
7b. If your game is cool enough, there can be a time limit that matters. This time must be the amount needed to escape from a very large explosion.
8. There is kissing somewhere in the game, whether it is sexual or sweet. It is usually given to the main characters as a reward for rescue. Observe the ending:
9. It is strongly recommended that players or characters be required to beat each other up in jest or out of competition. See above. If these options aren't available, consider making them able to hit each other at anytime.
10. If there is conflict between groups or a few individuals groups, the motives for that group are quite unclear and usually amount to basic despotism. Bosses and villains can only be memorable because of their moves or even their looks, but not for their motives or words.
11. Obstacles in regular levels that work by alternating between on and off must are optional. So are pits and holes. Alternating obstacles must deal in elements, crude substances or high technology.
Required, unless in a fantasy setting:
Gear + Powerups
1. The main method of replenishing life will not be first aid kits or anything having to do with technology. It will be food. Junk food gives you the least, appetizers, fruits, and vegetables give you a medium amount, and meat of any form gives you all, or almost all of your life back. This food can come from:
--Greasy guys you just beat up with your bare hands
--Revealing them by breaking hollow objects like barrels or boxes
--Out lying in the open, unattended, in the sun
--Revealing them by breaking large concrete items that are not hollow, such as statues, rocks, and signs
2. It is preferable that there occasionally be something for the player to ride on and hit people with. If there is, when you reach the end of an area, the screen goes dark and a new area loads, the ride must disappear. Enemies who are not part of number 8 or are not bosses can use these as well.
3. Shops are permissible, but only if the game is badass and has a basic inventory system. It is unfortunate that Double Dragon III broke this rule.
1. There must be at least one boss who is either completely mechanical or is a person relying on a machine. It is preferable to include both.
2. There must be at least one boss that is a giant mutant or abnormal or created creature of some kind.
3. At least one boss must have a bunch of regular baddies surrounding him. At least one must be able to call for reinforcements.
4. At least one boss will come back to challenge players again. This can be in different form or state if desired.
Think I covered most of it, though I'm sure people can point other stuff out. Jokes aside, CC is amazing and one of my favorite parts about it is how it combines the best of the old with a dash of the new.