In RTW the charge is very powerful and speed is king. The Gaul units will hit you en masse and cause units to rout in seconds. You can't present a long enough front using traditional Roman methods in RTW. Better to put tougher units on the ends, weaker in the center and present a long line. Otherwise you have two units hitting each of yours and they won't last long.

What you really need is for the line to hold long enough that cav can decide things on the flanks. You are giving up +4 attack to the AI on hard. Cav is overly powerful in RTW.