Question

one challenge geography provided romans

Answer 1

The Roman empire was huge. Without internet or phone or anything like that, it was extremely difficult to unify the entire Empire under one leader.
Imagine if every time your county's leader made a decision it took a month for someone on a horse to get to your city/town and tell you.
So many areas in Rome became their own city-states with relatively independent leaders, which wasn't what the Romans wanted when trying to build a unified empire.