 Prince Rupert of the Rhine , a prominent Royalist general , attempted to relieve Windsor Castle that November . Rupert 's small force of cavalry was able to take the town of Windsor , but was unable to overcome the walls at Windsor Castle – in due course , Rupert was forced to retreat . Over the winter of 1642 – 3 , Windsor Castle was converted into the headquarters for the Earl of Essex , a senior Parliamentary general . The Horseshoe Cloister was taken over as a prison for captured Royalists , and the resident canons were expelled from the castle . The Lady Chapel was turned into a magazine . Looting by the underpaid garrison continued to be a problem ; 500 royal deer were killed across the Windsor Great Park during the winter , and fences were burned as firewood . 
