"Ring of steel" was the term used for the road blocks, barbed wire and railings around central Belfast in the 1970s, when bombings there were common [1]. In this case it was literally steel barriers encircling the city centre. It was not a continuous ring of steel, as buildings formed parts of the barriers.

It's also the term used for the covert ring of CCTV cameras and checkpoints erected in the City of London [2] in 1993 in response to bishopsgate bombing. It shares the property of encircling the centre of a city for security purposes, but is not literally a ring of steel. At one point people would have recognised this as a reference to Belfast's situation.

[1] http://www.independent.co.uk/news/uk/belfast-security-measur...

[2] http://henriwilliams.blogspot.co.uk/2010/08/entering-panopti... http://www.independent.co.uk/news/londons-ring-of-steel-to-e...