The Lines of Communication were English Civil War fortifications commissioned by Parliament  and built around London between 1642 and 1643 to protect the capital from attack by the Royalist armies of Charles I. In 1642 some basic fortifications were built, in the form of street barricades and small earthworks. In 1643 a major construction effort was made, to provide a comprehensive ring of fortifications around the city. Much of the work was done by volunteer labour, organized by the trained bands and the livery companies. Up to 20,000 people are thought to be involved, and the works 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.
Question: Who were the fortifications constructed to keep out?
Answer: Royalist armies of Charles I