Both armies were similar in number, approximately 15,000, and both short of ammunition. The royalist army however, had better provisions having been able to take advantage of the supplies gathered in the town for the approaching parliamentarian army. In addition the royalist forces had the advantage of superior cavalry, in number, experience and ability. It was thought by some of Charles' advisors that Essex would not engage but attempt to sneak away in the night. Thus, whether through complacency or inefficiency, the royalists did not take advantage of their early arrival at Newbury to secure crucial areas of the terrain on the night of the 19th September.
Essex had no intention of retreating and during the night secured a crucial tactical position of Round Hill. Deploying troops and artillery among the many hedged and ditched enclosures of Essex also positioned men in the hedges to the south of the hill and drew up in battle array upon Enbourne Common or Heath where it joins Newbury Wash Common. The royalists had now given the tactical advantage to the enemy by allowing them to deploy first. In the morning, from their position on top of Round Hill, the parliamentarians bombarded the royalists artillery positions and the troops on the lower ground, paying particular attention to positions where they suspected the king to be.
The task of taking the hill was given to infantry under Sir Nicholas Byron and cavalry under Sir John Byron. The initial charge of the infantry resulted in heavy loses and so then the cavalry were thrown into the action. The fighting was fierce and lasted much of the day, with little ground gained or given. At one point the royalists gained the summit of the hill pushing the parliamentarians back into Skinners Lane and capturing an artillery piece. BUt they were unable to hold the position for long and, with the aid of reinforcements from the London Trained Bands, the parliamentarians took back the hill.
Fighting on the flood plain of the River Kennett to the north of the hill was also indecisive with both sides hampered by the small enclosures and neither able to gain the advantage. To the south, on Enbourne Common, Prince Rupert commanded the royalist cavalry. Here and along the plateau connecting the two principle scenes of action, the pattern of fighting was much as that around Round Hill; fiercely contested but indecisive. Both sides continued to bombard each other and fighting continued along the line throughout the day; neither side able to gain the advantage. As dusk began to fall both sides were exhausted and had suffered heavy losses with no ground gained or lost.
Twelve hours of inconclusive action must have made the royalists aware of the futility of fighting the same ground on the coming day ut, perhaps most importantly of all, they were running desperately short of ammunition. Under cover of night the royalists retreated towards Oxford. Expecting to resume hostilities the next morning Essex fired upon the supposed royalist lines to be met by silence.