The same rook-pawn fortress works on the a-file: the defending king toggles a8-b7, and the knight protecting a7 can never gain a tempo or dislodge it without allowing the capture.
Rook pawns are the knight's nemesis on either side of the board. White's pawn on a7 is protected by the knight on c6, and Black's king occupies the corner. The king alternates between a8 and b7, attacking the pawn whenever it is not protected by the knight.