Iron! It's simple. We dig Iron to improve weapons and armor of our minions! I'm afraid this will make some extra work for our dear 3d designers, but in overall, whaddaya think?
This would work like this:
you mine iron. Then depending on how much iron you have, you can adjust the number of updates made. each creature will have 2 upgrades, an armour and weapon upgrade.
Say, to upgrade 1 goblin armour you need 100 iron. you Have 10 goblins, so if you have 1000 iron, you can purchase 10 upgrades. You cannot however purchase more upgrades then the number of creatures for whom the upgrade is supposed to be made.
If the creature dies/leaves while the upgrade is being manufactured, the number of "to be manufactured" items decrease automatically. If the creature dies/leaves when the upgrade has been created, the upgrade will disappear, restoring some mana as a refund.
The armor value will adjust depending on the creature level, so no additional calculations would be needed. If the creature's level 1 armor value is, say, 100, then with armor it would be 130. Lets suppose, that with each level creature gets 15% additional percent of armor. So for unarmored creature on level 2 it will be 115 and for an armored one - 149.5. This will be calculated through all levels. So if you have a level 6 creature upgraded, you DO NOT count from the current level, but do the calculations from level 1 to 6. So for goblin lvl 6 it would be: 130(1+.15)^6 = 300.7