During the 17th century a great divide developed between King Charles I, and the Houses of Parliament over the rule of Kings and how Parliament should govern. This caused the English Civil War, and the execution of the King. Those on the King's side were called Royalists, and the others Parliamentarians. Many Parliamentarians were also Puritans, believing in a simple Bible based way of worship and life.

Sir Robert and Lady Brilliana Harley were Puritans and staunch Parliamentarians, living in a strongly Royalist county. They were in a minority, isolated in their home. 700 Royalist troops, led by Sir William Vavasour, besieged their castle in the summer of 1643, outnumbering the 100 men, women, and children inside. During her husband's absence in London, Brilliana led the defence of the castle. Four hundred of her letters to her husband and oldest son survive which demonstrate her powers of leadership. The village, including the medieval church, was almost completely destroyed.

The siege lasted seven weeks, but ultimately failed when the Royalists withdrew to Gloucester. Sadly, the stress of the siege undermined Brilliana's health, and she died in October 1643, leaving her young family behind. A second siege in 1644 was successful. The castle was taken, and her daughters led off to prison in Shrewsbury.

For many years, the English Civil War Society made biennial visits to Brampton Bryan to re-enact the Siege of the castle, sponsored by Aardvark Books. The most recent visit took place in August 2022.