KIng CHarles had taken too much power, which upset the Parliament as they wanted to have power too. But when King Charles rejected there want of power, Parliament didn't like this and started to rebel. After that the King gathered his men and declared war against Parliament. (this is in August 1642)
So when the war had started the first part of war was won by the Royalists and Parliament has lost a lot of men. Then the leader of Parliament, Cromwell decided he needed to train hjis men and have more advanced equipment. After hard training Cromwell's men were ready for battle, although their new improved skills and equippment was an advantage there was a large difference in numbers there was only 400 hundred parliamentarians to the 3000 Royalists. Cromwell had to win this and pressured his men to win. After a tough battle Cromwell won the civil war. Parliament had powewr again.