Celts have Wales as a core region, but any cities built there flips to the english when they rise. Historically, Wales was an independent region after the roman withdraw from britannia and have many kingdoms in the region until the conquest in 1283 by the Kingdom of England. So, my suggestion is to remove this region (the hill where Cardiff stands and the stone hill) from english rise of civilization flip. This way, the english will have to conquer the region and will give more action to their early gameplay.
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Another suggestion is to include the anglo-saxon raiders in brittania, spawning some axeman or swordsman inside boats in the east side of the island between III A.D. and V A.D. centuries. In my experience playing as the celts in 3000 B.C. scenario, they lose control of all cities in gaul to the franks when they spawn, only remaining the british cities. These ones have a quite peacefull and stable gameplay until the spawning of the english. Implementing this, can give more challenge to the celts gameplay and making more historical, showing the conflicts between the bretons, picts and the germanic peoples that have settled brittania