Considering that they don't even use them historically, at least not from Caesar's writings. Of course, the game starts way before Caesar, so it is possible that the chariots were used for shock value and eventually lost that use through becoming ineffective for whatever reason. I don't know enough about that to say for certain. Similar to how the "feared" scythed chariots were mocked by the Romans, the main experience Rome had against them they were ridiculously ineffective. According to Caesar, Briton chariots were used as armoured personnel carriers to get troops in and out of combat quickly.
I do think Britannia should get archers or javelinmen of some kind, primitives almost always knew the use of the bow and/or javelin in addition to the sling. The head-hurlers are darned unrealistic, heads were a gruesome kind of trophy so they would not be used as missiles. Unless somebody has knowledge that I don't, if somebody does, enlighten me, please. The lack of true cav is somewhat realistic, though. They had very little, from my limited knowledge, and certainly not enough to make a worthwhile unit out of.
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