Civilization V: The Celtic Chronicle, part 3 (520 A.D. to 1450 A.D.)
1140 A.D.: With the Picts streaming out of the woods like ghosts and arrows raining down from the walls of Edinburgh and Dublin, the astounding might of the French army is dashed like so much surf against mossy, kilt-wearing rocks. The queen who commanded the defense becomes known as Queen Alesia the Indomitable, and the day of the French retreat is declared Celtica Day. Many retreating French troops are cut off by carefully-positioned Celtic ambushes, and do not leave Republic soil.
You know those moments in gaming when you just pull something off that totally shouldn't have been possible? That just happened to me. It's impossible to convey the emotions of several tense turns of meticulously-calculated maneuvering, and the utter joy of finally watching the French retreat. I had no business winning this. Even on my home soil, I was outnumbered at least two-to-one, out-teched, weakened from fighting off the Russians, and mostly unprepared.
What I've essentially pulled off is defeating the largest professional army on the continent with a bunch of shirtless guys with pointy sticks. It wasn't easy, by any means, and I had to spend whole minutes each turn thinking ahead, anticipating French movements, and doing as much damage as I could to priority targets. I've lost Nantes, but they won't hold it for long.
1150 A.D.: The Republic Rangers, having held Dublin almost single-handedly, pepper the retreating French with arrows and wipe out most of what remains of their invasion force. The survivors fall back to occupied Nantes.
1190 A.D.: Nantes is retaken. This represents the height of the Celtic Empire, with the homeland and Celtic Britain being whole for the first time since the German Invasions. And yet, all good things...