Unit costs should not scale smoothly proportionally to the civ's tech level, but change relatively to it. E.g. there should be very few units in the earlier eras (done with maintenance costs), more in the industrial period and many in the modern -- thus, frontlines will appear. In the post-modern period, units should become expensive again. But powerful, to be able to take cities. In the ancient and medieval era, cities should be typically taken by long sieges (city under siege should not be able to work tiles and its health should decrease with starvation).