We need to add that 2x2 boss king!
Stupid brilliant ideas for a wacky variant with some random events

Football edition: Get the a piece into your opponents "end zone" to eliminate the player. Bishops, rooks, queens will be restricted in a way.
That's just racing kings.

The objective should be to beat the boss. All 4 players play as a team and wins if they beat the boss. The boss have 100 lives and whenever a piece tries to capture the boss, that piece will stand next to the boss instead and the boss will lose 1 life. The boss will have various attacks and movements. For example it can move down and capture any pieces there in the process. Another move is an attack in a direction which will kill all pieces within a 2x2 radius in that direction. The difficulty can be set by modifying how much life the boss have and what moves he have and how common those moves will be. How smart an AI is can also be a factor.
An additional rule could be that the winner is the player that dealt the most damage to the boss or that there is a real player that controls the boss.
As a side note, I think the boss should have some really powerful moves to make it difficult.
What happens if a knight attacks the boss?
more than 3 years after, the topic is still alive. i think the castle should be "3D"
and yes, we need an awesome JavaScript programmer... I only know python and a bit of java...
So, the castle should be made of a 4x4x5 base. a rook can stop on the adjacent square and then climb up(same for bishops and queens.) the player inside the castle can't see whats happening in the outside unless he gets a rook down or in mid point, on which he will be playing in the outside "fog of war" but with Amazon vision, independently of the piece.
for climbing with a knight, the player must land on an adjacent square to the castle and climb it up with knight moves. there are also the top squares, which the castle player knows
the player can also take a turn to "see" the side squares and can in that turn defeat any piece climbing or in the edge, but can't move. Note the castle is 3D, so the player(s) can place their pieces "over" others instead of capturing. Also, with this rule, its possible quadruple check, which should be worth 30 points with queen and 70 points with other piece.