The measures were introduced following a massive IRA bombing campaign in the City in the early 1990s such as the 1992 Baltic Exchange bombing and the 1993 Bishopsgate bombing. At this time the sentry posts were guarded by armed police almost continuously. Initially the ring of steel consisted of plastic cones and on duty policemen which the locals described as the 'ring of plastic'. It showed the public that the City authorities were taking seriously the threats by the IRA. This was replaced by more permanent structures consisting of concrete barriers, checkpoints and thousands of video cameras. After the IRA ceasefire the police guard was reduced.