The following rules were faithfully scribed by the beautiful Caitlin Hill, as dictated on the fly by two drunken duelists (myself and one Mark Cugini, who did an amazing job of hanging tough as I changed the rules to fit my needs throughout the game).
Required armament:
- 6 ping pong balls
- 3 large cups (towers)
- 2 medium cups (chambers)
- 3 small cups (turrets)
- 2 water cups (moats)
Basic Play:
Set-up is like beer pong, except players can build their own castles any way they'd like, provided each cup is at their end of the table, accessible, and stable (so it won't fall over if hit by a ping-pong ball) – players must okay the opponent's construction before the game begins. You must also agree on house rules for Peasant's Revolt, Knighthood, and the Sword in the Stone (explained below). A Whiskey Bonus Cup may also be decided upon at this time.
A possible castle construction, with turrets mounted atop the towers and chambers. Castle structures should be stable enough to withstand enemy fire.
Game Start – Each player gets 3 balls.
First shot is simultaneous – players maintain eye contact and do not look at the cups. If no one sinks a shot, players immediately begin a Barrage--rapidly shooting available ping pong balls until a cup is landed. If opposing cups are landed simultaneously, Barrage continues until two cups are landed on one side.
First to play gets to use remaining balls in play for his/her turn (i.e. if Player 1 sinks the first shot, Player 1 then shoots the remaining five balls).
Here, Player 1 has chosen to build a lovely fortress in the mountains.
Storming a Castle:
Unlike regular Beer Pong, certain cups require several balls to be "stormed," forcing the opposing player to remove the cup and drink the contents:
Towers (large cups) take 3 balls to storm
Chambers (medium cups) take 2 balls to storm
Turrets (small cups) take 1 ball to storm, same as traditional Beer Pong
The battlefield. Note the captured balls in the tower on the left, and the differing castle structures.
If a ball lands in a "turret" (small cup), opposing player drinks the cup and the ball goes into the water cup "moat" (to be used by opposing player when his/her turn begins). Balls sunk in a medium or large cup stay in that cup until the cup is "stormed." These balls are considered "captured" and are out of play.
Close-up of captured balls.
If a player doesn't get the necessary "stormed" total in a cup in one turn, the "captured" balls stay in the cup until the necessary total is reached (i.e. if Player 1 only lands 1 ball in a medium cup, it stays there, captured, until a second ball is landed in that cup, even over several turns).
Once a cup is stormed, the balls it contained are put back into play for the opponent's next turn, unless Knighthood is achieved:
Knighthood (optional):
If 3 balls are landed in a large cup in one turn, all 3 balls go back to the shooter and that turn continues. This is the only event that returns balls, except in the case of a "Peasant's Revolt."
Peasant's Revolt:
If all 6 balls are captured, all 6 balls are removed from the cups and go to the owners of the castles. However close a player is to storming a cup, the total starts back at zero for all remaining cups. Play continues for the player whose castle contains the most captured balls (so if Player 1 has 4 balls captured in his castle, Player 1 takes the next turn and shoots 4 balls).
Castle Pong: always goes down smooth.
The Sword in the Stone (optional endgame):
Loser shoots all 6 balls at any remaining cups on winner's side, and continues shooting until he/she misses. The winner drinks any cups "stormed," loser drinks remaining cups.
Yes, pour your beer, Mark--but not even your Griffin familiar can save you now.
I've mellowed a bit in my old age, but I'm still the one to lead the drunken, naked charge when the time is right. 
