Seems to me that St. Michael's Mount would be rather less difficult for a Cornish army to storm than the Tower of London: the song only makes sense if "Michael's Hold" is the more difficult task. A clerical author would also be more likely to use spiritual metaphors than a lay author. Given that it was written well after any actual risk of action taking place, some inflationary language is not surprising!

So despite doubts, I think I'm holding to the religious interpretation.