St Michael's Mount is a small island off the Cornish Coast. A stronghold I think.
Mont Saint Michelle in Normandy is a similar island which has a church and small walled village on it.
I think that the song refers to St Michael's Mount. Trelawney was a bishop so maybe he had connections with the church on the mount.
He was freed so the uprising was a waste of time but it's a cracking good song all the same.