The trick, if you can call it that, lies in the prefabrication of steel-and-concrete blocks that can be hoisted into position, just as bricks form a wall.
One of the most serious incidents occurred at O'Neil's Road in Newtownabbey, where about 100 Protestant rioters -- many of them teenagers -- threw concrete blocks, paving slabs, bricks, planks of wood, gasoline bombs and Molotov cocktails at police.