Iron and horses for example could be later required for buildings (that actually consume them, not like circus). Iron for example could be required for some [insert] building that gives some kind of significant production bonus. Plus maybe giving a bonus to production for factories (coal still being a requirement to build it in the first place) etc.
Same with horses, but different effects, say food (and maybe some happiness). Some [insert] building that consumes one horse as with iron and plus some food related building that gets a bonus to food etc.
As for the units...
I would further enforce the rock paper scissors 3+1 system and make it available from start. Crudely explained spear beats horse, horse beats sword, sword beats spear with arrow being ranged +1 that can damage all, but also can get wiped by all. That being the base available from start with all kinds of other hybrids and UU's mixing it up a little.
Anyway the idea is that the core 3+1 being available from start, and it's better gear that is upgraded upon tech progress and resource availability. Like I have 10 str sword (bronze) after bronze working, 12 str horse after horse riding and 9 str spear after bronze working (with arrow after archery of course).
AND after iron working and iron resource I upgrade my sword to 16 str (iron). Same later with steel, 24 str sword (steel). Same with horses getting steel > being knight counterparts etc etc.
Anyway that's how I'd do it, instead of not having a whole unit type available to you (infantry / sword).