Encounter at Midpoint and the Celtic Bronze Age
3200 B.C.: While scouting a barbarian-occupied ruin, a chance meeting occurs between the Celts and parties from the Russian and French tribes. They all eye one another warily, each wanting to be first to expel the barbarians and claim the secrets of the ruins.
3175 B.C.: The Russian chieftain approaches the Celts, asking for aid against their French enemies. The Celts agree to join the war on the Russian side, granted they are given time to prepare.
This was my first interesting diplomatic decision. Refusing to help would hurt my relationship with Russia, and I was planning to start a war anyway. Better that war be two-on-one, even if it does delay my conquest of England. Thankfully, Civ V allows you to request a 10-turn delay on a joint declaration of war, at which point I might finally have some Pictish Warriors ready to go.
3050 B.C.: The Celts master the art of combining tin and copper, ushering in the Celtic Bronze Age. They immediately begin using the new metal to make pointy objects of different varieties.
Yes! Now I can start churning out Pictish Warriors and advancing my faith toward the 300 needed to found a World Religion. Being the first to do so will bring me great advantages through the early and mid game. I'm now switching my research to try to build the historically Celtic Stonehenge wonder, which will give me further bonuses to Faith.