Thinking further about the Nuclear Weapon method discussed above, I might simply make all 3rd & 4th era units require charcoal and I think that'll sort out the resource issue; I could save the nuke method for some elite fourth era units; perhaps some sort of training camp/special barracks? Or alternatively, give someone like the ancient med civs a chance to bring out a couple of bonus units, but they need to build the SW first? Just some thoughts...hopefully you'll have some ideas!!