Puzzle #17: Themeless
Sharpen your pencils, because I have a 21×21 themeless for you, and it isn't going down without a fight. Today's puzzle was largely inspired by this marvel of construction by Erik Agard. I chose a less wide-open grid pattern than he did, but I hope the result is similarly challenging and fun.
For the real sadists, I've provided a puz file with Down clues only. (Good luck.)