This is an example of Roman rigidity.A few years after the Wall was complete,it was realised that goods/cart traffic through the Wall,would be better concentrated at easier crossing points,so milecastle gates were reduced in width, to accommodate this directive.In this case,the south gate has been reduced in width , so carts could not cross here,largely due to the watery/boggy nature of the ground immediately to the north of the Wall AND A STEEP DROP JUST BEYOND THE NORTH GATE.