I don't see the argument here for adding bronze, at least as far as using it to create units goes. The only viable unit for it is the spearman. You can make it common enought to be ubiquitous, but in that case it might as well be infinite, which is essentially what we have right now with spearmen not requiring a resource. It might be nice for mining purposes, but not for a unit.
Agreed. We don't need Bronze to return imo., Spearkman / Pikeman is supposed to be the alternative you go for if you don't have Iron to build Swordsmen. Spearmen/Pikemen should be units you go for if you want to stay defensive (because you generally get bonuses when being on defence) whereas the Iron units should be stronger and allow for an offence.
The problems arise from the facts that:
a) If you skip Swordsmen, you can get Pikemen not very much later than you would have gotten Swordsmen. Apart from their upgrade path, Pikemen are better than Swordsmen.
b) Longswordsmen come too late to really have a significance.
c) You don't really need Swordsmen to take cities, because your melee units will just be meatshields while Crosbows/Trebuckets take down the city.
I would suggest the following changes:
a) Lower combat strength of Pikeman from 16 to 15 or even 14. Pikemen are supposed to be anti-mounted units, and with their 100 % bonus against Knights (20), they will still be strong at 14. Lowering their strength will make the jump from Spearman to Pikeman less in strength, but given that they go from +50 % to +100 % mounted bonus, the difference will still be noticable (Spearman has a an effective 16 against mounted, Pikemen would be 28).
b) Reveal Iron at Mining and give Swordsmen a +25 % or even +50 % against melee units. This will give them a more defined role in combat.
c) One could consider moving down Longswordsmen to Metal Casting (or same level as Metal Casting)? If that was done, one would probably have to shift some techs around to secure a gap between Swordsmen and Longswordsmen, but the advantage would be that Longswordsmen come at the same level as Pikemen, and that the gap between Longswordsmen and Musketmen becomes bigger. One might lower the strength of Longswords by 2-3 (perhaps 19?) but if one retains the +25 % against melee, they would probably still be more useful than currently.
d) Give Archery units -25 % strength against Cities and let Siege Weapons start with Cover I promotion. I mentioned this before, but CB should
not be your default unit to take down cities, and Catapults generally suck in their current form.