The High Road - an unconventional retro challenge

Before I present the problem (a construction challenge), I just want to say that this is a very special one for me, from a personal standpoint. I am by no means a serious problemist, and my own compositions tend to either be fatally flawed in some obvious way, or turn out to be mediocre at best (by casual standards, that is; this is neither the time nor the place to speak of professional standards).

But I can honestly say that this particular puzzle perfectly embodies pretty much everything I genuinely value in the field of retrograde analysis - a few examples would be strangeness, the underrated merit of illegal positions, and avoidance of convenient tools and tricks that artificially increase complexity.

And lastly, I've never come up with a problem of even barely acceptable quality in less than an hour (and possibly never in less than 2 hours, for that matter). Yet somehow this one just "came to me" over the course of less than 30 minutes... almost as if by destiny. I'm virtually 100% certain I'll never produce a legitimate composition in less than an hour [or two] ever again. In fact, I may never produce another legit composition, period, as such a thing typically requires many days and hours.

...Anyway - on to the construction challenge.  The goal is to create an illegal position, but to do so by taking what I'd call the "ultimate high road". More precisely, the position must also satisfy ALL of the following criteria:

• Each side has exactly one king. 
• Neither side is in check.
• Neither side has a queen.
• Neither side has 5 [or more] of any single piece type.
• With the sole exception of the kings, no two pieces of the same type may occupy squares of the same color, unless the pieces themselves are also of the same color. For example, no White rook can be on a dark square if there is also a Black rook on a dark square somewhere on the board.
• The board configuration alone must prove illegality. So you may not specify whose move it is, castling rights, or any other additional information about the position.
• The final restriction is more or less the sole thematic motivation for the challenge. It could probably be described in as few as 2 words, but is given as a full sentence in the comment below. Before scrolling down, see if you can guess what it is...  ;-)

[Note: the 5th requirement was added much later, in light of more recent developments]

The final restriction is that there must be no pawns on the board. Anyone who correctly guessed that [without looking] is already thinking on the right "wavelength" and will most likely complete the challenge.

Here's an example of what NOT to do:

Just posting to congratulate Remellion on having been the first to complete the challenge. Quite fast, as usual, and his position was every bit as pretty as I'd expected a correct solution would be.

Great work, and thanks also for not spoiling anyone else's fun, in case others are currently working on it too (as I suspect they are).  :)

I have a solution as well. Post it or not?

Edit: there are of course many, but I am trying to find one with a free unit out in the field.

Currently 2 free units, going for 3. Sorry, meant 3, going for 4.

I had 3, would be impressed if you could find 4 (or prove otherwise).

I guess I'd prefer a personal message, if you don't mind.  :)

I'll wait and try to get my 4 free units. I have no good tools to get diagrams in messages. Have to work that out one day.

^ You could send a FEN string (along with a brief proof of illegality). 

4 free units! Don't need either the black rooks or the white rooks! They can be anywhere on the board!

Good idea. I'll put together a FEN-sting. Have no FEN-generator.

^ Oh, my - it must be something I'd never have thought of.

That's a good thing since I just discovered a tiny glitch in it . Back to 3. I'll ponder a little while longer and then send you the best I've got.

dex trasee vool rue

What I meant was that free units for both sides would imply you'd built a proper pawnless retro-cage... something I'd have happily bet my entire life savings was impossible (mainly because if it was possible, then some other gifted professional would have published it long ago, and such a composition would surely be too famous for me not to have read about it).

Of course, if you DID manage to achieve that, it would be far more impressive than even the version you sent me with 6 free units of 1 color. In fact, it would be a great enough feat for me to [strongly] advise you against sharing it here, or in any other public forum for that matter. There'd be too much risk of someone stealing your idea and publishing a composition based on it... and perhaps even winning a serious competition you ought to have won. 

Arisktotle just sent me a valid position with 6 [almost] free units. Of course, he decided to be somewhat "cheeky" by omitting these pieces from the diagram without stating the [9] forbidden squares. So I sent a "cheeky" response in which I applauded his skill and effort, but also included a proof game for his position + 4 Black knights + 2 Black rooks (1 on a forbidden square).  :D

Actually, there is no almost. Freedom in this context means the presence of the 6 units is not required to set up the temporary cage. That they shouldn't be in some spots is irrelevant from the construction task viewpoint.

IMO, that is!

Such a composition would certainly violate Gödels first incompleteness theorem and that is not a place I want to go on this forum!

The Cobra has left us but I've kept his memory alive on a vacant spot in the diagram:

Okay, so both Remellion and Arisktotle did some excellent work here, and smashed the heck out of my construction challenge! 

Of course, I realized quite some time ago that the original version of the problem was a little too easy, as there were a rather large number of satisfactory positions. After some serious thought, I came up with a harder version that should pose some legitimate problems for solvers. I shall list the additional restriction below (all of the initial criteria must still apply too, obviously), and then later edit the 1st post to reflect my revision.

The new stipulation is as follows: With the sole exception of the kings, no two pieces of the same type may occupy squares of the same color, unless the pieces themselves are also of the same color. For example, no White rook may be on a dark square if there is also a Black rook on a dark square somewhere on the board.

If anyone manages to solve the updated edition of this challenge, I'll be very impressed!  :)

Disappointing; just one free unit