Taunton Castle grew from the long association between the town and the bishops of Winchester. An Anglo-Saxon settlement and defensive center existed at Taunton before the Norman Conquest, but Bishop William Giffard is traditionally credited with beginning the stone castle in the early twelfth century. The complex developed around a great hall, gatehouse, walls, domestic ranges, and administrative rooms. It served less as an isolated frontier fortress than as a residence and management center for the bishop's extensive Somerset estates.

Successive bishops repaired and enlarged the buildings while royal officials occasionally used them during periods of unrest. Taunton's markets, roads, cloth trade, and agricultural hinterland made the town more important than the castle's modest military record might suggest. By the later Middle Ages, parts of the complex were used for courts, storage, hospitality, and estate administration. Alteration and neglect gradually transformed the original plan, yet the gatehouse and great hall remained prominent elements within the town.

The English Civil War returned Taunton to national importance. Parliamentarian forces seized the town in 1644, and Colonel Robert Blake organized its defense against repeated Royalist attacks. Sieges in 1644 and 1645 brought bombardment, street fighting, hunger, fire, and heavy destruction across Taunton. The castle formed part of the defensive position, but walls, barricades, houses, and churches all contributed to resistance. Relief by Parliament preserved a western stronghold whose survival disrupted Royalist operations in Somerset.

After the Restoration, the castle's military role faded, but it remained a place of law and punishment. Following the failed Monmouth Rebellion of 1685, Judge George Jeffreys held proceedings associated with the Bloody Assizes in Taunton's castle hall. Hundreds of prisoners across the region were condemned to execution, transportation, whipping, or imprisonment. Later rebuilding adapted the complex for county courts and institutional uses, leaving medieval fabric embedded within structures shaped by Georgian and Victorian requirements.

The Somerset Archaeological and Natural History Society acquired the castle in the nineteenth century and developed a museum there. Major modern work created the Museum of Somerset while conserving the great hall, gatehouse, surviving walls, and archaeological evidence. Taunton Castle is therefore not a single-period monument. It records episcopal lordship, urban administration, Civil War resistance, judicial terror after Monmouth, antiquarian collecting, and public museum use. The surrounding town is equally essential, because the sieges were fought through inhabited streets rather than within a detached castle enclosure alone, affecting local civilians as well as soldiers.