WordNet sense 1 (able to withstand attack):

inviolable, secure, strong, unassailable, unattackable

WordNet sense 2 (immune to attack; impregnable):

invulnerable

WordNet sense 3 (capable of conceiving):

conceptive

WordNet sense 4 (capable of reproducing):

fertile

WordNet sense 5 (incapable of being overcome, challenged or refuted):

inexpugnable

WordNet sense 6 (not capable of being conquered or vanquished or overcome):

unconquerable