Simplify: Roman military engineering was very good in its time. So they were able to build a double-wall of fortifications that was thirty miles long in total (both walls together) in just six weeks, when they wanted to encircle a besieged city in Gaul.
Roman military engineering was pre-eminent amongst its contemporaries, and the scale of certain military engineering feats, such as the construction of a double-wall of fortifications 30 m long in total (both walls combined total) in just six weeks to completely encircle the besieged city of Alesia in 52 B.C..