Instead, the Romans built a gigantic ramp of rock and earth extending from the nearest adjacent mountain so they could simply march up to the level of the walls.
It took 2 years for the Romans to build a ramp.  
mnli hypothesis: It took 2 years for the Romans to build a ramp.   premise: Instead, the Romans built a gigantic ramp of rock and earth extending from the nearest adjacent mountain so they could simply march up to the level of the walls.