It seems to me that what you have is right. These three words describe the enemy, and why they were just about to do a runner.
commoti (perf pass part of commoveo) having been excited/agitated/disturbed
summo terrore (abl) by the greatest terror.
So the literal translation might be 'having been disturbed by the greatest terror', but to put it into smoother English, sometimes you have to move not just words, but whole phrases around:
The enemy were absolutely terrified when they saw that the Romans had built a bridge ...