Garde has not the to keep meaning but the to guard one, so it’s more like be on (your) guard (literally "take guard", i.e. take care, pay attention).

e.g.:

• Prends garde à toi (take care of you)

Beware that prendre garde often keeps the same meaning regardless of whether it is used negatively or positively.

See Prendre garde : expression à plusieurs variantes

Garde has several meanings https://www.larousse.fr/dictionnaires/francais/garde/36093

What interests us is the last definition :

• En boxe, en escrime et dans les sports de combat, attitude que l’on prend pour engager le combat et se protéger.

Which means Combat Position.

You can understand Prends garde has Take a position that allow you to defend yourself.