Beginners watch rows and columns and get killed on the diagonals: a five counts in ANY of the four directions, and the diagonal is the hardest to see because the eye reads lines and columns, not slopes. Black already owns the main diagonal from (6,6) to (9,9) — that is a four, with both ends free. Finish it: either end completes five, and white cannot hold two doors.
Finish this step to move on.