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Romans invaded Britain in AD 43 and, chasing ancient Britons along Thames, they came to first place where it was easy to cross. They built garrison there – and London was born. They also erected bridge over river. Garrison became major trading post. Later, bridge suffered neglect and whole area was raided by Vikings. In AD 886 Alfred Great drove out raiders, bridge was repaired and city prospered again. Hundred years later, Vikings returned but King Ethelred sailed up Thames, attached ropes to London Bridge, headed downriver and pulled it down.
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They built garrison there _______.