Besides many other jobs assigned to him by God, St Michael is the foremost door warden. As such, his churches and chapels may often be found on doorways to the Otherworld. Therefore, whenever you find some building dedicated to St Michael, it is worth while investigating its history. It might stand on one of the doorways to the Otherworld.
It’s the Gospel according to Nicodemus that first portrayed St Michael as the door warden of Paradise. This Gospel is accounted an apocryphal part of the Bible. Apocryphal means, that man has taken it upon himself to edit the word of God and decide what God has said and what He didn't say (it is the most blatant case of cherry picking you might encounter). Apart from his day job as porter of Eden, St Michael fulfills many other requirements and aspects attributed to all bouncers of the Otherworld. They include keeping track of good and bad deeds of souls, weighing and judging souls immediately after death and on the day of the last judgement, and accompanying them to the Otherworld.
If some of this reminds you of ancient gods and goddesses, that is no accident. Already in the first Book of Henoch (another apocryphal part of the Bible), these latter attributes are added to bring St Michael on a par with pagan counterparts like Thot, Anubis, and Horus (in Egypt) or with the Zoroastrian Immortals Rashnu and Sraosha. It is hardly surprising that he was found the right person to take over from his peers such as Nodens, Charon, or other door wardens in other religions.
Doorways to the Otherworld were famous and dangerous places often called The Isles of Glass. A well known Isle of Glass is the Mont Saint-Michel in the Normandy region of France, the place where King Arthur killed a giant. It was logical that only St Michael could be made the patron saint of such a place, filling in at the same time as door warden for the locals and as Prince of the Seraphic Host to hold it against unholy influences from the past.
Later, the order in charge of Mont Saint-Michel founded a dependency in Cornwall, and not surprisingly, this place is another of those Isles of Glass, St. Michael’s Mount in Cornwall, or An Garrek Los y’n Cos. He place was most probably known already to the Greeks and Romans by the name of Mictis or Ictis.
A further famous Isle of Glass was Glastonbury with its Tor, on top of which only the tower of the St Michael’s church remains these days. Another famous Isle of Glass may be found in Skellig Michael in Ireland.
But the Otherworld could be reached by other means than going to the Isle of Glass. All over Europe, there were caves that were reputed to be entries to the Otherworld. One of those was in the region of Apulia in Italy, where the Normans built St Michael's Church in a place called Monte Sant’ Angelo integrating the cave into the church. It says on the doorway that all sins are forgiven to whoever enters the cave, a clear indication that the cave is the forecourt to afterlife.
If you dig into the archaeological history of a building dedicated to St Michael, you will often find that there were earlier occupants to a holy site, be it a temple, a shrine, or a stone circle. It is one of the many ways by which Christianity integrated the locals into the new religion coming into fashion.