A Sudoku with only four given digits and some more rules

This is one of the hardest Sudokus (and one of the best Sudokus) I have ever seen, with only 4 given digits. There are some more rules too:

1. First, normal Sudoku rules apply here.

2. Both the diagonals also should contain 1 to 9 in some order.

3. You cannot put the same number in cells which are at a knight's move (in chess) apart.

4. The center box will be a magic square.

Now comes the real part, can you solve it?

Bonus: Is the solution unique, or can you find any other solutions?

This is not a puzzle of my own devising. However, to avoid spoiling the solve for others I will wait to declare its source until the puzzle is solved. Credit goes to Mark Goodliffe and Simon Anthony for solving this puzzle, and also to Aad Van de Wetering for making such a wonderful puzzle !

Edit: This puzzle got solved here - https://www.youtube.com/watch?v=hAyZ9K2EBF0

• Very nice puzzle! I can place like 11 numbers than I am stuck. Looking forward to other's solutions.
– daw
Oct 9, 2020 at 11:03
• @Ananymous. Is it Simon Anthony? Not Timothy
– DrD
Oct 9, 2020 at 12:56

A tough one. Spent much time on it. I can also confirm that the solution is unique (assuming I didn't make any mistake).

For the beginning, I only give some snapshot of my progress. Some of the reasoning are a bit complicated. But everything is logically deduceable until the last step.

At this point I got stuck and cannot see a possible logical solution. So I looked at the circled grid below. It must be either 7 or 8.
Assuming that it is 7, I arrived at the following situation, where the two grids with question marks can neither be 9 now.
Thus the circled grid must be 8.

• Welcome to Puzzling.SE! Another user already answered this question correctly back in 2020 and provided their full workings, rather than just the final step as you have done here. Please be careful not to repeat answers that have already been given. You should also hide your answers in spoiler tags >! to avoid spoiling the solution for anyone who wants to attempt the puzzle themselves. May 8, 2022 at 17:17