The factor that most contributed to the development of roman civilization was

The factor that most contributed to the development of Roman civilization was their well developed political system coupled together with their disciplined army which was gaining victory after victory lead to a great success in the development of the Roman civilization. 

They made a checks and balance system to equal power within the branches. the "check" meant that one branch would watch one branch and "balance" them if they were too powerful. if you want more info on this, i would just google checks and balance system.