I just love Imperivm. Btw I only recently realized how that games shares an important gameplay feature with another prototypical RTS: warhammer.
in warhammer there are NPC units called "creeps", that must be killed by your heroes in order for them to gain experience, because hero experience has great value in that game, and could be compared to certain resources in AoE or 0ad. All this also holds for Imperivm, where NPC enemies hold common settlements which have various uses, and you can fight them as soon as you have an hero with an army that's good enough, first of all in order to level the hero, and then maybe employ the settlement if you need it in your strategy.