the gauls the worst defensive tribe in game
The phalanx produce more anti cavalry per minute than the praetorian. Gaul are good at quickly raising a defensive force, and judging by the littles simulations i've done, they also seems to have the best of the three wall (not counting natar ofc).
In my eyes, the roman are the worst defensive tribe. Also don't underestimate the paladin anti infanterie production coupled with the fact that he is a quick raiding def unit.
To answer you initial question, i would play roman if i wanted to build a hammer without the need of spending gold. Mace outshine everyone in term of offence production but they eat so much.