Simplify the following text for a Grade 9 native English-speaking person.
Original: 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. .
Simplified: 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 .