Most crosswords have "seed words"—words/phrases that crossword constructors try to use first, as the basis of the remainder of the puzzle.
George knows a great deal about opera, as does Martin, though to a lesser extent. But it is safe to say that we are both classical music buffs, with George having a particular expertise in opera.
George thought that ROBERTO DEVEREUX, a well-known opera by Donizetti, might make a good seed entry. Also, this entry seemed to be promising for stacking, since it had a decent number of vowels, used fairly common letters (which eventually help with Down words), and had a great "X" at the end. While X is a rather rare letter to find in a stack, words like SEXES, TAXES, TEXAS, etc. make it easier to use in that position than one might think at first glance.
We interrupt this midrash to say a few words about bel canto in general, and this opera in particular. Robert Devereux is part of Donizetti's so-called "Queen Trilogy"—the other two being Anna Bolena and Maria Stuarda—and the great sopranos of the world going back to Maria Callas (and including ... never mind, let's not inadvertently slight anyone by providing an explicit list ...) seek out these vehicles to demonstrate their artistry. George remembers seeing the incomparable Beverly Sills tackle all three queen roles with the New York City Opera in the early 1970s, a feat that landed her on the cover of Time magazine (look familiar?). The Metropolitan Opera Company has Roberto Devereux in its 2015-2016 repertoire. But any doubts as to the appropriateness of this entry ought to be removed by listening to this amazing pastiche of eleven sopranos interpreting its finale.
OK, we now return you to our tutorial in advanced crossword construction: It seemed to us that the X of DEVEREUX meant that the best position for this entry was the second position down in a potential quadstack. Therefore the next word needed to be somewhat "see-through"—which is a made-up allusion to vowel-rich entry beginning with a vowel and preferably ending with an E. Towards this goal, ONE_ON_ONE_DEFENSE worked perfectly. Look at those two 15s, and see how well they create good Down-word possibilities. We got lucky, or so we thought!
The top 15 was expected to be the the hardest to find, at least not until we knew what the bottom 15 might be. So saving the hardest for last, we focused on the bottom, and independently noticed that a nine-letter word ending with STATES might do the trick. After chasing a few dead ends, we hit upon PERSONAL_ESTATES. This seemed to offer good possibilities, but easier said than done. The ?DEL in the dead center suggested author EDEL, with an E in the middle of the top 15, as an obvious choice.
So far, all of this had gone relatively quicky by quadstack standards. Now the real challenge began, and this is what makes this genre so much harder than triple-stacks. Even though several potential entries were promising for the top 15, we had no diagram to speak of, which gave the feeling of being stuck in the middle of some crossword desert!
To make a long story short, we found CHOLESTEROL_FREE as the top 15, which made for a rather cool-looking quadstack.Still, integrating it into a workable grid requires much resourcefulness. Construction software (we both use Crossword Compiler) can take one only so far. Autofill is of little use towards the arduous task of finding moderately long down words that run through the stacks while obeying the rules of crossword symmetry. It took a good 2 to 3 weeks of rather diligent work, that included all sorts of blind alleys and "close-but-no-cigar" traps, to find SOBER_MINDED and THE LEFTOVERS, and build smaller words around them.
Alas, this not the end of the story. The opera that had been our seed entry was unfamiliar to two editors, and another editor suggested that ONE_ON_ONE_DEFENSE was not nearly as common as MAN_TO_MAN_DEFENSE. We'll concede the point, but our original entry also Googles well enough. What do you think? Watch this YouTube clip while making up your mind.
In preparing the puzzle for self-publication on our pages, we decided that ONE_ON_ONE_DEFENSE, clued in basketball fashion as "Man-to-man court ploy," covered all the bases [to stretch a metaphor]. This was a perfect collaborative effort, in the sense that each of us contributed an entry that ultimately had the puzzle rejected by the MSM!
P.S. There's another opera allusion in the puzzle, with the clue for ICE. And one of our friends pointed out that SEXES, which crosses ROBERTO_DEVEREUX at the X, anagrams to (the Earl) of ESSEX. We would love to claim that this was all part of the plan, but that would be an exaggeration!
P.P.S. Note too that the puzzle has NESS at 9-Down. Now ordinarily, this is clued either for the Loch (or the monster allegedly therein) or the "Untouchables" agent Elliot. Our clue is the sneaky "Stack part." That's Robert Stack, who played Elliot Ness, but it kind of winks at this puzzle's trademark Stack!