As you progress through the game you need to upgrade your main base to further unlock buildings, units and technologies. Your first upgrade to your main base is to the "Ancient Age" (level 2). "Classical Age" is simply the next upgrade (level 3). The cost to upgrade from "Ancient" to "Classical" is 30,000 gold, although you will have to upgrade your gold stash to allow yourself to carry that much gold. The price to upgrade gold stash and units to acquire 30,000 gold may far surpass 30,000 gold, as unit and upgrades cost additional gold. So expect to gather more than 30,000 gold in the long run. Jewel cost shown is price to speed up the upgrade, i think it starts at 70. Achievement will pop when upgrade is completed.
I suggest upgrading your gold stores to more than 30,000 capacity and grinding out the gold through battles. Once you have the gold, use it, no one can steal it if it's being used.