The following sections treat the basic concepts of Castlingmost, followed by the specific rules of two forms of Castlingmost Chess.
Those who like the game may call it by its nickname, which is OOmost Chess (pronounced Oh-oh-most, sounds a bit like Kokomo).
Of course, you cannot make a move that consists of Castling two enemy pieces with each other, and when you Castle with an enemy piece, your piece is considered the primary -- the primary piece moves further when there an odd number of squares in between.
Please notice that because all moves are Castling moves, but a King may not Castle out of check, a check may easily be checkmate; however, one may Castle with something else to interpose a block, or one may Castle with the piece that's giving check to remove it from its attacking square.
What if you Castle with the enemy King? For example, 1. Ke1-d2+d2-e1, d7-d4+Kd2-d5 check. I believe this must be allowed because it makes the game more interesting; and after all, consider the purpose of forbidding the K to castle in cases involving check -- surely these rules were never meant to apply to cases where you can move the enemy King!
In fact, this example is forbidden because it is a null move. Instead, a WP on a2 may Castle with a BP on b2, trading places; and in response, Castling a2 with b2 is of course forbidden because it is an "undo" of the move that just happened.
The even number of squares may be 0, 2, 4, or 6; and
The piece that initiated the Castling is the "primary".
Two answers are presented here, but others are also possible.
For example, after 1. Ke1-e2+e2-e1, Qd8-e7+e7-d8 gives Check. The Q cannot really capture the King, but it looks like check, so it is check!
The idea of this game is that all the power is always on the board, which should cause a tense situation. In fact, since Pawns can be promoted, and since promotion is the only way to gain a material advantage, the armies get stronger as the game goes on.
Not only do the armies get stronger, but the board also shrinks! If a1 becomes empty, no piece can ever move there again; and after a1 is empty, emptying b1 creates another dead square! Thus, as the game goes on, more power is concentrated in less space, until at last it explodes.
I haven't examined the game much. I think that games will be long except when blunders happen -- but it will be very easy to blunder.
1. Ke1-e2+e2-e1, Qd8-e7+e7-d8 check 2. h2-e5+c7-f4 Qe7-e6+e5-e7 gives check again, but is a risky response because it brings the WP to its 7th rank. In reply, 3. a2-c4+Qe6-b3 is forced but 3...Qb3-c4+c4-b3 is check again and b3-c4 is an illegal "undo". 4. c2-d3+Qc4-c2, Qc2-d2+d2-c2 and you can see that because it is forbidden to repeat a previous position W is mated in a couple of moves.
It is possible that a move such as 1. Qd1-e2+e2-d1 forces a win. I haven't analyzed it.
The strategic idea of this game is that movement becomes ever more difficult as pieces are captured; and if you are stalemated, you lose.
I haven't examined this game at all. I think it should be both playable and enjoyable.
Compare it with a piece that moves a Ferz but captures as Chancellor, mFcRcN. Its ideal value would be the same as mRmNcF, and perhaps their practical values would also be equal -- the mFcRcN has enormous capturing power, but its minimal movement makes it difficult to aim it at anything; and the mRmNcF can hope to use its great mobility to amplify its tiny capture power by forking things.
Whhether these pieces are actually worth a Rook, or more, or less, is one of those questions of piece values that I have not investigated. Maybe someday somebody will.
The point is, in OOmost Chess, all movements are the same, and pieces differ only in their ability to capture (or to give pictorial check). The mFcRcN would be enormously more powerful than the cFmRmN in OOmost Chess.
Armies whose pieces have capture powers same as movement powers should be able to have an even game of OOmost Chess, and most existing armies fill the bill.
This is one of those cases where the idea of the game is so interesting that it really doesn't matter if it ever gets played.