WordNet sense 1 (not capable of being surmounted or overcome):

insurmountable, unsurmountable

WordNet sense 2 (incapable of being overcome or subdued):

invincible, unbeatable

WordNet sense 3 (impossible to surmount):

insuperable

WordNet sense 4 (impossible to subdue):

indomitable, unsubduable

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

inexpugnable