Each legion had a squad of engineers, sappers, and various kinds of craftsmen called fabri. The studies indicate different numbers of these departments. Historian Paweł Rochala (“Las Teutoburski 9 rok n.e”) states that there were 120-160 of them in two centuries. In turn, Andrzej Grackowski (“The Roman Army in the Decline of the Republic: organization, weapons, tactics”) gives the number of about 100 people without specifying the number of centuries. There you can also find a thesis that separate fabri units ceased to exist after Marius’ reforms, and qualified legionaries were to be assigned to individual infantry units. However, while I support this author’s views on numbers, I doubt whether the engineering divisions as such have been liquidated.
The commander of fabri was prefectus fabrum. He was a man from an equine environment. He must have been a man with a great deal of knowledge in the exact sciences. One of the well-known prefecti fabrorum from the period of the end of the republic was Marcus Vitruvius Pollio – the author of “On the architecture of the ten books”. The commanders of the engineering troops belonged to the strict staff of the legion commander.
It should be noted that since the reforms of Marius, each legionary carried tools for engineering works, which increased the “engineering strength” of the legion, because legionaries could thus perform most of the fortification, siege and construction works, requiring only professional supervision.
The engineers were also responsible for the construction of the bridges. The army built three types of bridges for its needs. If the current of the river was not too fast, boats were used to cross the current. The second type is a bridge built on twenty supports made of stone blocks, the upper part of the bridge was made of wood. The third way to build was to build a bridge entirely of stone.
