Gaul is the best for defense focused style. Not just because of best resource/defense and defense/time, also the most important movement speed. While phalanx is the backbone for major battles, druid/haeduan though a bit expensive, they allow you arrive on time in most case to compansate their cost.
Roman is generally a poor choice for defense focused style. Praetorian's defense/crop is great indeed, but its movement speed is terrible. You have to position them near some vital village, eg. treasury or major off village. Or the chance to be arrived on time is way too low. Legionnaire could be medicore troops to defense compare to phalanx, and its train speed is way too low. But it can use as medicore attack troops. Not recommended to make too many of them, it is crop inefficent and it's def/time is very bad. One worse thing about Roman defense is, it does not have defensive cavalry, which means it's single village output is far lower than another two tribe and you can not utilise ally flag/shoes to make the arrival chance bigger. Also, you will need to build more villages to compansate it's low troop/time.