They are one of the easiest factions in the game, their isolation allows them to expand against rebel provinces with no threat of invasion from AI factions. They can become incredibly wealthy (with both Britain and Ireland conquered) and then just sit there and max out their troop recruitment options and general stats by visiting the 'island of darkness' woowoo. I invaded mainland Europe with two full stacks of high end units and laid waste to everything, first I all but destroyed the germans (they attacked me first!) and then turned on the romans who were advancing into gaul, wrecking their legions with glee. I thought they would be a big deal but my fear generating units, Belgian swordsmen and chariots made short work of them, routing them back to their Italian borders. Of all the factions I have played barring rome and carthage (babies first factions) they are the easiest. Your heartlands are all but secure (naval invasions are a rarity) and you can just sit on a pile of gold doing what you please... initially they seem a little intimidating but so do many factions. Casse should be downgraded to a moderate challenge... very difficult my ass.