What are the cataracts of the Nile?

The cataracts of the Nile are shallow and stretches between Aswan and Khartoum where the water's surface is broken by numerous small boulders and stones that lie on the river bed, as well as many small rocky islets. This is called "rapids or "white water". River cataracts often go with a rapid down-drop of the river.

Explanation:

first, the romans stayed isolated because they had to defend their own territories from neighboring cities that were harassing them, like the etruscans. second, once the romans managed to defeat their neighbors, and started to expand their territory, they started to build roads and bridges. finally, now that the romans have covered their territory with roads functioning as highways, they could mobilize their legions faster to assemble larger military units in a specific place.