Originally Posted by mr ploppy
It's the arch-shaped hole between 2 adjacent terraced houses that leads into the back gardens. It's private property (shared by the 2 houses), so alleyway or ginnel doesn't seem to fit.
You are making it too hard.
It is just an alley entrance to the back and whatever is there.
Up in the Northeast in the USA it is common for houses on narrow lots, but usually only in the suburban areas are they open or with a gate. In the cities, they would be locked or usually bricked or walled off from the outside for security.