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During the English Civil War the majority of Londoners supported the Parliamentary cause. After an initial advance by the Royalists in 1642 culminating in the battles of Brentford and Turnham Green, London was surrounded by defensive perimeter wall known as the Lines of Communication. The lines were built by an up to 20,000 people, and were completed in under two months. The fortifications failed their only test when the New Model Army entered London in 1647, and they were levelled by Parliament the same year.

What was the defensive wall that encircled London in 1642 called?
the Lines of Communication