Is this position legal?

Well done daydreamy! White can castle (the knights and pawns even spell OK.) The point here is that if white can castle, black cannot have promoted the rook - it must be an original rook, let in by white's captures axb3 and bxa3 (cross-capturing).

I'm out of original material, but since there seems to be some interest here, might as well post something.

Werner Keym
Die Schwalbe, 1979

Don't think it is legal...

The king might have sat on f7 via a6-b7-c8-d8-e8-f7 but after going to f7 the knight has no way to get into e8. d6 is the only way which checks the king forcing it to capture the bishop.

I don't see any other way.

"No, it's not legal" is almost never the correct answer in a retro.

It looks like the white's first capture had to be a rook, and the second could be a rook or a queen.

Still not correct, Frankwho. Your starting position is illegal.

I actually nearly worked it out but haven't had chance to test it. Unfortunatly someone beat me to it (i did figure the above out indepentantly) if you substitute the Lower rook with a queen (i.e. not the one on the 8th rank) it might work, but i'm not sure?

Summersoltice is correct. The last move must have been white Bg8xQh7. Full logic as follows, to illustrate the thoughts that go into these problems.

White's king is behind black's pawn chain, so it must have entered from a6-b7-etc. But black's knight needs to come to e8 after wK has crossed e8. At that point the wK cannot be on f7 as bN needs to come in Nd6-e8, and d6 would be check. Therefore wK must have been on g8/h7/h8 when the bN came in, and only after that the bK walked in via h7-h8.

From the diagram position, what was the last move? It can't have been bN from d6 (illegal check), and it can't be b6-b7 (the wK needs to have come in via b7.) Therefore the last move must be white's bishop from g8 to h7, possibly a capture.

If Bg8-h7 was not a capture, again black has no last move (bKh7-h8 is impossible, as the king would be in check on h7 and white would have had no way to deliver this check.) Therefore Bg8-h7 must have captured a black piece. Not a knight (bN must have come from h6, which is illegal check) and not a bishop (no last move again). So either a rook or a queen.

Now we ask: how did a wB get into g8/h7? It can't be from g6 (black clearly played h7xg6 so that line is closed), nor can it be from f7 (wK needs to play Ke8-f7-g8-f7 as we established earlier, and it happened before the bK got to h8, which was before wB got to g8, so that line is blocked.) Therefore the wB is promoted via h7xg8=B after white's king went f7-g8-f7. What was captured on g8? Not a bQ or B (check), not an N (again illegal check) so it must have been a rook; h7xRg8=B.

So one of black's rooks was captured on g8. The other couldn't escape from the corridor along a8-e8 because of the black pawns and the immobile bishop on f8. Black has all 8 pawns, so didn't promote to a rook. Therefore the piece the wB captured on h7 was not a rook either, so it must be a queen. Therefore the last move must have been wBg8xQh7. Before that black's move was Qh7 from somewhere, and the position is "clearly" legal. Summersolstice gave a sample proof game.

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Composers generally don't give illegal positions. Retro composers in particular make positions where the point is that the position is legal but must have involved very specific moves/sequences no matter how you try to reach the position. The following one might be on the very tricky side of "easy" - can you find a game leading to it?

Veisberg, Y. & Ya'akov, A.
Fairy Chess Review 1948

Yes; last move was Bxc3 (moving from b4 or a5, capturing anything). It's also easy to extract the bishop from upper-right so that gxh6 can be undone.

I don't see why a retro puzzle can't be illegal. It can at least keep you honest. But of course it would reward the lazy people who don't feel like trying to solve it as well, true.

It's not that it can't, it's just that hardly anyone wants that to be the answer (composers or solvers).

@Garrus_Vakarian: Legal. Move Nb1 and Bc1 out of the way, put Black's Q on b1, put Black's K via b4-a3-b2-c1-d1-e1, put everything back together. Same with the part at the top.

#1 is illegal

Took ya long enough :P

Actually, if people are really looking for something difficult, then illegal positions are actually harder. It is easier to prove something can happen than proving that it can't happen.

I think it is possible. A queen might have been promoted and checked at f4 with the white queen on the f-file too.