Synonyms for Unclimbable (same or very similar meaning)
WordNet sense 1 (incapable of being ascended):
unscalable
WordNet sense 2 (incapable of being surmounted or climbed):
unsurmountable
WordNet sense 3 (incapable of being passed):
impassable, unpassable
From the ODE community, based on WordNet
add/edit