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A stone barrier 74-miles long, up to 15ft high, and 10ft thick: Hadrian's Wall stood as the Roman Empire's most imposing frontier for 300 years. Almost 2,000 years after it was built, archaeologists have properly excavated less than one per cent of it, but they have unearthed extraordinary findings. Julian Richards journeys back in time to unlock its secrets. 4/6.
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