After both sides castle kingside, a very specific danger appears: if a king's own pawns are still on their starting squares, the king has no escape square on the back rank. A single rook or queen landing on that rank, with nothing to block it, can be checkmate on the spot.
This is called a back-rank weakness, and it's one of the most common ways games are decided even between experienced players, usually because both sides were focused on the middle of the board and forgot to look at their own back rank.
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Black's king has no escape square, its own pawns block every one.
Which move delivers checkmate?
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