So, basically every unit does get a new ability when upgraded from copper to silver to gold. And this is really important, because that is partly what is makeing gold units strong. The problem with centurions is that in a normal battle the only thing they get compared to silver spearman are the stats, the ONLY occasion where i saw the Impenetrable Defense from the centurions do something was when they were lvl 9 and i was fighting the lvl 10 leviathan in the tactic school shoting at me with this giant cannon, besides that i didnt ever see even just 1 centurion dropping his shield(if the ability triggers they lose the shield). And having such an ability just doesnt make them worth using. Thats why i would like to see them getting something else. Here just 2 ideas.
Centurions are trained to fight together. They take X% less damage from melee attacks.
(maybe increasing a little bit if you use 2/3/4 Packs of Centurions)
Centurions are using a gigantic tower shield reducing ALL normal(not magic)dmg dealt by X%.
I would really love to see something happen regarding this!