The initial (Shaffer's) main point of limited resources and units they fuel was the creation of some (rare) elite units vs. numerous mediocre units. While those elite units could be seen as the terrifying center of an army, all the common units would be the body, the replaceable masses.
In vanilla Civ this worked quite well: Swordsmen and longsworsmen where the rare elite while spear- and pike-men could be seen as rank and file. It worked because:
1) the strength difference was remarkable enough and
2) the spear-men line finally upgraded into the infantry-line
G&K, while all in all a great improvement, messed this up a little bit by dividing both lines in two parallel strands with all to equal strength.
A solution should achieve two goals:
1) Re-create the feeling of elite units that need iron as compensation for their power
2) Prevent a player without iron from missing an entire era-upgrade
Moreover I would add as an additional goal:
3) Sharpen the different unit-line's profiles
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To solve this challenge, my idea was to create some additional units. They should be available with the same technologies as the elite version, possess lesser strength but don't need iron. Then I read the thread again and I realized that this proposal:
I think being able to add an "upgrade" to melee units is sensible. Reinforcing their armour/weapons what not at the cost of an iron. The addition to strength should be percentage wise. I really like that idea - maybe a mod could be done of it?
… is exactly what I wanted to achieve – but it was simpler. Gucumatz, I don't
like your idea... I
love it!
Anyway, you may find totally new names for the new units or you may just name them “Swordsman” and “Elite-Swordsman” - here are my proposals:
– Classical era --
Spearman

56

09
(-2)
Abilities:
+100% bonus vs. Mounted,
break attack *)
Peasant (or light swordsman)

62

12
Swordsman (or elite swordsman)

75, Iron

15
(+1)
Horseman

75, Horses

11
(+1)
Abilities: Move after attacking, no def. bonuses, -33% vs cities,
+33% Str. on open terrain
– Medieval era --
Pikeman

90

14
(-2)
Abilities: +100% bonus vs. Mounted,
break attack *)
Light Infantry (or light longswordsman)

100

18
Longswordsman (or elite longswordsman)

120, Iron

22
(+1)
Abilities:
free promotion: siege (OPTIONAL; might be too much!)
Light Cavalry

110, Horses

16
Abilities: Move after attacking, no def. bonuses, -33% vs cities, +33% Str. on open terrain
Knight

120, Horses
+ Iron

20
Abilities: Move after attacking, no def. bonuses, -33% vs cities,
+33% Str. on open terrain,
break lines **)
Musketman (for reference only)

150

24
*) new ability „break attack“: Prevents cavalry's “move after attack”
**) new ability (free promotion) “break lines”: may chose all adjacent hexes after attacking an enemy to move on – even behind the attacked unit.
The two new abilities where introduced to “sharpen the unit's profile”, as I mentioned above.
a) The “break attack”-ability is quite intriguing, as I think. Currently, the anti-cavalry units are not able to hunt their targets down. They are simply to slow to do so. I read a book recently, where a cavalry-attack against a group of pike-men was described: The riders were “immobilized”, their attack was broken and they were bound in a close fight.
b) The additional cavalry-strength on open terrain was introduced to compensate for the just described indirect degradation. And because I feel that the higher maneuverability in appropriate terrain should be reflected somehow.
Another possibility would be an +33% strength increase after moving at least one tile before the attack. I would love this, too!
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Seeing iron with bronze-working is quite a good proposal, IMO.
Additionally it might be a good idea to decrease the usual amount of iron per deposit and increase the number of deposits while equalizing their scatter on the map.