After serving throughout Europe, Rosworm settled in Manchester in 1642 where the town engaged him on a £ 30, six-month contract to lead their defence against the Royalist Lord Strange.
Despite an approach from Strange to defect to the Royalists, Rosworm completed the town's makeshift fortifications on 23 September 1642.
Now made lieutenant colonel in Ralph Ashton's regiment, he organised the fortification of Preston, following its capture on 9 February 1643.
[2] Rosworm was in action in the assault on Warrington and in the fortification of Liverpool before returning to Manchester, then under threat of attack by the William Cavendish, 1st Duke of Newcastle.
In reply to his repeated petitions of parliament, in 1651, he was rewarded with a post in New Yarmouth to oversee preparations for defence against feared enemy landings.