Civilization V: The Celtic Chronicle, part 2 (1150 B.C. to 520 A.D.)

T.J. Hafer at

Rise of the Celtic Republic

80 A.D.: The English army flees York for London as the advancing Celts encircle the city and the PC Elitist peasants rise up against them in the streets.

90 A.D.: English bowmen try to outflank the besieging forces at York, but are intercepted and wiped out by Pictish reinforcements from Nottingham.

115 A.D.: The Celtic Council of Freemen gathers at the Edinburgh Conclave, formalizing the establishment of the Celtic Republic.
Becoming a republic will increase my production even further, which should help me quickly make the switch from warfare to development once I conquer England.

Later that year, York is captured. The pious, PC-worshipping Englishmen, like their brethren in Nottingham, are allowed to elect their own representatives to the new Celtic Senate and maintain local rule.

The English monarchy are now reduced to the city-state of London, which prepares what forces it still commands to repel the inevitable Celtic advance.