Woops, did I create a challenge too difficult already?
Ok I will change the rules a bit, a check is allowed, for the bonus challenge, no checks are allowed!
Still don't have time to participate so I will just leave this here for now, will comment with my attempts / thoughts later
Good gravy shoopi that was difficult.
Fortunately I already found a position - No rooks, no checks and I don't even need to specify side to move, although it looks messy. (Also, should be sound...)
Remellion, I might be missing something, but I don't think that position is legal. I can't see how the Black king got to f2.
Kh4 -> Kg3 -> Kh2 -> Kg1 -> Kf2?
Kh4 -> Kg3 -> Kf2 might be quicker ... 
What is a "rook promotion"? Is it a promotion on a or h file, or is it a pawn, that promoted into a rook?
In reaching the position I gave, there must have occurred a promotion to a rook at some point.
The black king reached f2 from e1, and before that from f2 and g3. The unlocking is rather convoluted because I did not expect the cage to get so messy. As a starting point, consider what the (only) possible last move was.
Thanks, I see it now. Wedging the White king and queen in afterwards was hard to see.
Black has had two promotions. One of them must have been the bishop on g1, since the bishop on f8 must have been captured. What is the proof that Black's other promotion was a rook, and not a knight or bishop?
Remellion, that is beautiful, this is the sort of thing I was looking for when creating this thread. I like the retrocage a lot, especially the strategy in the south-west corner, as well as the general restrictions in the position to make it all work. I will provide the analyse if anyone is interested:
1) While Black is not in check, he has no legal last move, therefore it must be white who moved last.
2) What could have been white's last move? Obviously not Bh2-g3, nor b3/b2xa3 because then you have to retract the dark-square bishop. So that leaves either g3xf4 or Kb2(x)c1.
3) If the last move was g3xf4, the only entry point for the black king to reach f2 would be after white plays b3, then from a3-b2-c1 etc.. however if white plays b3 before a3, there is no way the black pawn can reach a2, therefore it is impossible. The last move is indeed Kb2(x)c1.
4) If the last move is Kb2-c1, black's knight still cannot get out due to illegal check (nice touch). So the last move was Kb2xc1. Which piece did it capture? It cannot be a bishop or a queen, since they have no legal way of getting there. A knight suffers the same fate. So the last move must be Kb2xc1R.
5) If the rook that was taken on c1 is original, white must have played axb and bxa to allow entry, as entrance from the kingside is blocked. Now it's time to count - in this scenario, white captures hxgxf, axb, bxa, and finally Kxc1, for a total of 5 captures. Black has 2 useless missing pieces (the original bishops) so the bishop on g1 is promoted from h7. This leaves black with 4 available missing men, which is one short of the required 5. Thus we have concluded the rook captured on c1 cannot be original.
6) Could the rook that was captured on c1 be promoted? the answer is yes. White plays a3, black plays axbxa2, white plays b3, black plays cxb-b1=R, black also plays hxg1, for a total of 4 white captures. White is missing exactly 4 men (2 knights and 2 rooks) which all can be valid captures, so it is just enough. Meanwhile, in this case white only captures a total of 3 black pieces (including the rook on c1) which leaves black with 1 extra piece, which is captured without a specific goal previously. The last move is Kb2xc1R. White king goes back to c3 and the entire cage can be released. Excellent position!
Well, this was a huge success, even if short lived.
I will post a new challenge if I think of something. Meanwhile, If anyone would like to, he could post the next challenge as well. It could be anything - even if it's easy, we could always make it tougher later on in the thread.
I have a challenge: Construct a position where one side has 2 knights, where a particular one must be original, and the other must be promoted. Preferably without checks.
Thoughts on the previous challenge: That was really hard to make. Glad you liked it. The relatively long analysis for a simple task was because all my simpler implementations failed. My strategy was to force ~xR as the last move via retropat, with the R inside a pawn cage. However, tries like [wKb1, wNa1, wBc1, wPa3 b2 c2 d2] and [wKa2, wBc1, wPa3 b2 b3 b4 d2] all failed because of illegalities or the lack of a retrocage for the bK; hence the awkward inventory-based retrocage finally. Another strategy I didn't attempt would be to have black's pawns on the 7th mostly intact and a cage with unlocking ~xbR the key to force promotion, but that probably is also tricky to construct.
Another strategy I didn't attempt would be to have black's pawns on the 7th mostly intact and a cage with unlocking ~xbR the key to force promotion, but that probably is also tricky to construct.
This was my original thought as well, however, very short trial and error shows that it is also tricky to create.
Your solution was my idea of a perfect strategy, where everything just falls into place. Also, I do not think the long analyse is bad whatsoever, quite the opposite, the more there is to say about the position, the better. That being said, however, I don't think it's THAT long, when explaining a retro puzzle step by step (things that are obvious to us), it takes a lot of text
As for your next challenge, awesome! I will try it as soon as I have time. However I'm afraid I might be able to cheat this time, since I think I have an old creation that is very similar to the requirements!
...You already have the positions actually - the only mechanism I could imagine when coming up with that challenge was one I saw you implement before. Nevertheless, I'd be interested to see if anyone can find a different strategy, or a more economical one.
Yes a more economical solution would be an improvement. I will try to tackle this later today after work.
Here's a least economical solution (fixed):
And this is what I have for economical (you might recognize some themes here):
I figure there is no point in making a new thread, so I'll post the next challenge here: Create a position where neither side is in check, there are no bishops on the board, yet a bishop promotion must have taken place. This should be easier than the rook challenge I think, but still fun.
The economical one is very elegant. I didn't expect the Nc8/Pb6 structure to come up here! Least economical was what I had in mind as a brute force solution.
B promotion was much easier. It's easier to build and fill cages with rooks than bishops. I tried an economical setup with the bN/wK thing again.
Excellent. It could be possible to make an even more economical solution, however, it seems like it will not be as aesthetic as yours.
Since we had rooks, bishops and knights challenges, I think it is only fair to make a queen challenge. Once again, same rules. Construct a position where there are no queens on board, neither side is in check, yet a queen promotion must have taken place. I think this time a slightly different strategy needs to be implemented, however, it might be an easy challenge once more.
Slightly different. This was my only idea to ensure the Q promotion. Again somewhat inefficient.
I don't think the knight version of this task is at all easy, so instead a different challenge: Construct a position where one side cannot castle because the rook involved is promoted. I haven't tried it yet, so it may be really easy or really hard.
Wower took me a LONG time cant canstle neither side 100% legal
White and black cant castle because both of there rooks are pormoted!
We actually had two of these threads in the distant past, which brought to life some pretty epic ideas, positions, innovations and failures. So here I am trying to revive this thing a little bit.
Can you create a position where neither side is in check, and there are no rooks on the board, yet a rook promotion must have taken place?
Note: I haven't even attempted to do this, so I have not the slightest idea if this is extremely easy or very difficult. The ball is now in your court, good luck (I will participate myself later, got to sleep now).