causing fear or anxiety by threatening great harm; "a dangerous operation"; "a grave situation"; "a grave illness"; "grievous bodily harm"; "a serious wound"; "a serious turn of events"; "a severe case of pneumonia"
15
impregnable
able to withstand attack; "an impregnable fortress"; "fortifications that made the frontier inviolable"
2
awful
causing fear or dread or terror; "the awful war"; "an awful risk"; "dire news"; "a career or vengeance so direful that London was shocked"; "the dread presence of the headmaster"; "polio is no longer the dreaded disease it once was"; "a dreadful storm"; "
1
inaccessible
capable of being reached only with great difficulty or not at all
1
unconquerable
not capable of being conquered or vanquished or overcome; "a tribute to his courage...and his unconquerable will"- R.E.Danielson; "faced unconquerable difficulties"