Could the entrance to Jacobs’s temple be considered a Priest’s Hole?

I think it is. Definition of a Priest’s Hole: a secret chamber in certain houses in England, built as a hiding place for Roman Catholic priests when they were proscribed in the 16th and 17th centuries. It appears that no one other than Jacob and Richard knew of the entrance to the temple.

I only mention this because in the Key to Time: The Stones of Blood episode while the Doctor was searching for evidence linking Vivian Fay to the Druid goddess they stumbled unto a priest hole. There they discovered 3 paintings spanning 150 years all of Vivian Fay.