blue wrote:Give SAT a try, if you like.
I used it with Mike Metcalf's 225x225 sudoku(s?), and it worked well.
However, the puzzle in question, from 2009, was lost when the Sudoku Programmers' Forum disappeared. In case it's of any further interest, I've used my (now faster) program to remove a few more clues, the result is attached. It's solvable with basic methods, but a slog.