Turrets were placed between milecastles,two turrets equally spaced.The turrets were constructed in stone,before the main body of the Wall was constructed.Even when in the first instance,it was decided to build the Wall in turf,the turrets were still built in stone.Later,when it was decided to build the Wall in stone,the turrets had flange walling built to either side at the preferred width of 10 Roman Feet.This was fine until the builders discovered that about 5 miles west of Birdoswald fort,is a geological feature known as The Red Rock fault,and west of that line is no more limestone,which was the preferred stone used for constructing the Wall.So in many cases ,as the fault line is approached,the Wall was reduced in width to 8 Roman Feet.Beyond the fault,the Wall was built in red sandstone.Sadly,due to stone robbing and the sandstone weathering badly,none of the red sandstone wall now exists,above ground.
see http://www.hadrianswallcountry.org/wall.asp