Mont St-Michel is a medieval city:  construction started in the very early 8th century in what is known as the Carolingian period when Europe was clawing its way out of the Dark Ages.

Its lanes are steep and narrow because defense (against invading forces) was a very high priority.  If soldiers are forced to advance up this street there are few places to hide and they can only be four-across at most.

Mont St-Michel has never been successfully invaded against resistant defenders.