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Do not blockade Londinium until the Senate offers you a reward for doing so. Blockading ports will greatly weaken the Briton economy, but if you're struggling with finances you may want to stick to land for the short campaign.
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Recruit more missile troops to wear them down before they charge.
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Once Gaul is defeated, your cities are relatively happy, and your armies are back up to full strength, all you need to do is conquer provinces until you have fifteen of them.
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