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I am glad it is posted here.

What exactly is this supposed to add to the game compared to orthodox castling? It seems to me it would be strategcally foolish not to castle to a square one step away from the corner. Standing in the corner is in general a weakness, as your King defends one less Pawn, and has fewer escape squares. Standing further away from the corner leaves a gaping hole n the undefensible side of the King. If in a certain position it is not allowed to castle the maximum distance, because you would castle through check, it would be suicidal to castle in that direction in the first place.

So the only application I see of a variable King destination is that in Q-side castling one now will always go to b1/b8 in stead of c1/c8. But to get that, the much simpler symmetric castling rules as in Janus Chess would suffice.

The variable Rook destination might have some use, although this is largely spoiled by the fact that we are not allowed to give check. (After all, this would be the main reason why you need the Rook immediately in a certain position, rather than allowing an extra move for it.) Usually castling is done in a game stage where there are no open files yet, so the position of the Rook is rather indifferent. So why not put it always next to the King?

It seems to me that this adds very little to Chess, other than complexity we could do without.

One can place the rook in an attacking position in the appropriate file - so it is not just a defensive move but an offensive one. Rather than make two moves and then move the rook to the center file. With one step castling one can move the rook to the center right away.

Well, as I explained, I cannot imagine who would ever want to use other King destinations than those to b- of g-file. Adding moves that are not attractive to use, will hardly lead to more variation in play. You can make variants where it is allowed to remove your own pieces from the board, in stead of moving them, or cpture own pieces, or teleport your King to e4 in stead of castling, but it will essentially stay the same game, as no serious player would ever consider it.

I have a strong suspicion that this castling rule flls in the same class: strong players would virtually never use the extra possibilities.

See diagram 1: 
Modified castling conditions: 
1. can castle and check opposing king. 
2. king must be next to rook after castling.


white to move:

castles e1-d1;a1-e1 //check



This scenario can apply other files as well. 

Can be even used in 8x8 std chess with interesting results. 

Strong players will not use b and g file always. 

Offensive minded - will castle with rook to the file that  poses the biggest threat to enemy. 




Free Castling as described gets the best of both worlds by moving the king to corner and moving the rook in the center. 

However, I prefer the version I proposed above as in Birds and Ninjas.

In the 1970s I read an old english descriptive notation book that talked about free rochade. The rules for free rochade said the king could go to the B, N, or R square, and the Rook could end up on the other side on the N, B or K/Q sqare. I remember the author explaining that it free rochade was the original French castling rule, and that it explained the popularity of the King's Gambit. When White can choose whether to place his King on N1 or R1 and the rook on B1 or K1, possibly with check, the King's Gambit becomes extra fierce. Because of this the author claimed castling was restricted to its modern form. I only saw mention of free rochade in one book, and have never heard of it since, but the King's Gambit explanation is quite plausible. Anyone else hear of free rochade, or know the book? Sadly, I read many books from the library as a teenager, books that are no longer published or wanted.