Old units are excellent under hereditary rule. It's a course of action that you can take before adjusting the culture slider. Just throw your old useless units into a troubled city for the.
I think the upgrade cost is a little high. Actually I wouldn't mind it, but the AI's cost is half. It's rather annoying that their entire army gets upgraded in a couple turns.
Well, it is half, thanks god!! It was set at 15% of the human player cost in warlords.. that was game breaking. Now, it is just a very good bonnus, and the AI doesn't use it automatically unless it has lots of gold, as a wise human player would.
To be truthful, lets say upgrading units is the alternative to use an economic system (commerce and all the infraestructure asociated with it) in favour of a production system, whereas you have better developed one system or the other will make you rather take this alternative more or less. Also, it is a formidable way to put up a competitive army from obsolete units in a rush (marathon speed being specially sensitive for this matter as raising an army might take for too long in times of need/unexpected war).
The trade off between gold and hammers being upgraded is precise to me, keeping it's consistency with the buying option enabled with Universal S, it also adds a small price to make veteran units and succesive upgrading meaningful (ie: if you upgrade a phalanx to pikeman and then to rifleman it will cost you a bit more than upgrading the phalanx directly to rifleman, to account for the standing of a competitive "in between" pikeman).
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I was surprised to see that they are double normal while the game speed is three times slower. That accounts for my mistaken impression above. In my defence I did "guess" the upgrade costs for marathon. A spearman costs 70 hammers (35 in normal) and a pikeman costs 120 (60) so the difference is 50 hammers and so the 170 gold you mention would breakdown as 50 * 3 gold + 20. So the upgrade formula holds in terms of hammer costs, my mistake was to assume hammer costs were in line with game speed. I have learned something today
A Combat 3, Flanking 2 gunship should be rather fierce even if he does cost 350 gold to upgrade. Much better than the ordinary 5 exp point gunships from barracks and Theocracy.
Riflemen have been replaced by SAM infantry when I researched Rocketry and I haven't researched Assembly Line yet. Nationhood is still a great civic with the +2 