Antonyms of adj defensible

1 sense of defensible

Sense 1
defendable, defensible -- (capable of being defended)

INDIRECT (VIA invulnerable) -> vulnerable -- (susceptible to attack; "a vulnerable bridge")

Similarity of adj defensible

1 sense of defensible

Sense 1
defendable, defensible -- (capable of being defended)
       => invulnerable (vs. vulnerable) -- (immune to attack; impregnable; "gunners raked the beach from invulnerable positions on the cliffs")