The
Achilles defense (following the
Naming Conventions by Andreas Kleinhans) may be helpful when it comes to a corner fight and looks like this:

When the attacker continues with a front attack then you can close up the long side directly.

Now it is important that the short side remains safe (use the
help lines to check it!).

Otherwise this defence makes no sense.
Note: Of course White may choose to attack the Achilles defence in other ways (see the following two pictures).


But these variants are not considered here (maybe later elsewhere).
Conclusion:
When White is attacking the upper side of the board and no other pegs are in the way, Black may try the Achilles defense if the attacking peg is in
zone A of the picture below.
If White attacks from zone B then he will be able to win the short side directly or with a Counter Achilles (in some cases after a Counter Achilles after a Counter Achilles...).
An example for a Counter Achilles after White's
j12: