White to move
White's king has an intact pawn shield, while Black's was shattered by White's sacrifice of a knight.
Black's h7 pawn appears to be a target, but Black has the diagonal leading to it blocked with a pawn that is well-defended. White could try to create a battery along the h-file with a rook lift, but Black has the resources to find additional defenders along the seventh rank.
Black's bishop is prepared to take up residence on c6, while his heavy pieces might all find their way to the g-file. With light-squared bishops on the board, the White king will be less secure on h1 than the Black king on h8. The pawn on f5, currently serving a defensive role, may be able to move to f4 should White play g3.
Black has a slight material edge due to White's sacrifice earlier in the game. Black's pieces are better coordinated than White's. Despite superficial appearances relating to Black's shattered pawn shield, his king is more secure. Moreover, Black has a decisive advantage.
The game continued 21.Re3 Ng6 22.Qh6 Qd4 23.Rh3 Qg7
Black's move is the only one that does not lose. 23...Rf7 loses because the h-pawn in pinned. 24.Qxg6+-.
White to move
24.Qxg7+ was best 24...Kxg7.
25.Qf3 is better.
The final error. 26.Qg5 allows White to play on even though his position is irretrievably lost. 26...Be6.
Black to move
Black spots the mate in four.
In Elements of Positional Evaluation: How the Pieces Get Their Power, 4th ed., revised and enlarged (Milford, CT: Russell Enterprises, 2010), Dan Heisman offers vulnerability as a positional element to replace the pseudo-element of king safety. A key element of vulnerability that Heisman addresses is strength of the forces the guard a vulnerable piece.
The king is absolutely vulnerable because he may not be sacrificed. An attack on the king must be met. But, in the illustration above, the Black king's relative vulnerability is low because there are more pieces defending than attacking.