Antonio Olmos photographs the walls built across Northern Ireland's capital city as a means of defusing sectarian tension. There are 99 of them, dividing nationalist Catholic neighbourhoods from loyalist Protestant ones. Some of the walls date from the early years of the Troubles, but an estimated one-third have gone up since the IRA ceasefire in 1994. Now, 'peace gates' are being opened in some walls in an attempt to foster greater links between communities