Mine or shoopi's?
Legal or Illegal (yes I know it's old!)
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Maybe I'm missing something obvious, but there doesn't seem to be much getting in the way of the position being legal. White could have set pawns up on c3, d4, e3, at some point play f4, black plays ...Ne5, and white plays fxe5. The bishop can get to h3 via e2-g4-h3. The queen can move along some path like d1-d3-c4-d5-d6-c7.
The black pawn on g5 is trickier of course; it seems that it must be the case that white's only captured piece, his bishop, must have been by the f6 pawn, which then moved to the bishop's place on g5. I don't see why the dark bishop couldn't have slowly moved to d2, e1, h4, g5 and get captured. Obviously black's queen didn't need to be in a rush to get to h4 or anything.
As for the rook giving check, white's king could have been on g1 a while ago and the rook on e1 or something. (Don't mind the side variations; I was experimenting and should have deleted them)
Legal. If it's chess 960.
Elubas wrote:
Maybe I'm missing something obvious, but there doesn't seem to be much getting in the way of the position being legal.
You then give reasons as to why it is legal 
. I intended it to be illegal because of castling through check, but as you said, the King could have been on g1 and the Rook could later have given check. I did make it up as I went along and it was my first attempt, so I hope I didn't do too badly 
.
Don't worry, it's ok. It takes time and experience to make really nice ones.
By the way post #52 is legal.
@hoynck: I thought the name of the thread was 'legal or illegal'. Shoopi's question was every time legal or illegal and I did not find a solution. Hence, my answer is illegal.
I know retrograde problems and am curious if you will find a way to solve the puzzle. Success! :=)
About my last puzzle, here's a hint:
It's true that black does not have any white pieces to capture. But white, on the other hand, has black pieces to capture.
The zig-zag capture is a common theme and one that is very good to know in retrograde puzzles.
Wonderful, you are right, the position is legal. :-)
How about this?
Whites last move was queen side castling and it is a well known mate in four after 1. ... Kg2! 2. e3 Qb7 3. Nb1 Qc6 4. Qa1 Nd3#
Well, yes - in order for the position to be legal white's last move must have been queen side castle. The question is, is that possible?
Tip: If you can't figure out what's problematic about a position, try to reach it.
I can not recreate the position. The problem is in the black squared bishop of black. In order to be set free must the pawn at f6 have captured the rook on h1. There are only two pieces of white which are captured and the other one is the white square bishop. That will never end on f6. Hence it must be the rook and the rook must leave its position using the f-line. That implies that the white squared bishop must have moved requiring g3. The rook can only come to f6 via e3 and for that must he be able to reach f3, requiring that the f-pawn is played to f4. If those are at that position prior to the capturing of the f-pawn, then is it for the black squared bishop of black impossible to travel from f8 via h6 to b8, because it will be blocked by the pawns on f6, f4 and g3.
No way that that bishop can arrive on b8.
Excellent.
Now the fun begins, as there are more possibilities in the position you haven't thought of.
In the last puzzle I showed you the zig-zag capture technique. This can be applied in this position as well, as black is missing two pieces, allowing possibly gxf and fxg. So this should give you more things to think about.
Good job.
If you're interested in more of this stuff, check out my new composition here:
http://www.chess.com/forum/view/more-puzzles/an-original-retro-problem-10
legal cause em passente