The "eye of the needle"

I read it was a narrow space in a wall with several posts in the middle
configured so that only people on foot could enter. No wagons, and camels
could not sashay around them. A defensive point. I would be interested in
seeing an actual archaeological remnant of one to understand it better. I
had heard the low entry wall, as well as a knee knocker thing. (Navy term
for shin buster on the door you had to step over.)

Steve