impregnable(of a fortress or other fortified place: able to withstand all attacks, see also: impenetrable; inconquerable; unvanquishable) adjective [UK: ɪm.ˈpreɡ.nəb.l̩] [US: ˌɪm.ˈpreɡ.nəb.l̩]
inscrutable(difficult or impossible to comprehend, fathom or interpret) adjective [UK: ɪn.ˈskruː.təb.l̩] [US: ˌɪn.ˈskruː.təb.l̩]