In about 75 AD some 500 Roman auxiliaries, attached to the Second Augustan Legion at Caerleon, built a fort at Caerphilly. Garrisoned through to the middle years of the second century, it was sited alongside one of the main legionary roads, and lay approximately halfway between forts at Cardiff and Gelligaer. Following the abandonment of the fort, for over a thousand years the site appears to have remained unoccupied.

After the invasion of 1066, the Normans gradually expanded their territory across much of the country, including into the fertile coastal land of South Wales. King William built a castle at Cardiff in 1081. However the Normans remained near the coastline, leaving the defensive uplands of hilltops and wooded valleys of the north in the hands of the Welsh.

By 1263 a large area of land in South Wales was controlled by Red Gilbert de Clare, as he was known after the colour of his hair, Lord of Glamorgan (1263-95). Gilbert played an influential role in English politics at this time supporting Simon de Montfort and his reforming party of barons in their campaign to reduce the power of the King. However he was soon to switch allegiance. In 1265 Prince Llywelyn ap Gruffudd was rewarded for his support of de Montfort with recognition of his title of prince of Wales and of his authority and lordship over the Welsh magnates throughout his principality. This proved too much of a threat to many English barons who had land in the Welsh Marches, and in 1265 de Clare joined forces with Lord Edward, the future King Edward I. They confronted de Montfort at the battle of Evesham, where Earl Simon was slain and his army defeated.

De Montfort's son and many of the dead earl's supporters fled to his Warwickshire castle of Kenilworth. Here de Clare joined with the attacking royal forces, observing at first hand the impressive system of defences, in particular the water defences which surrounded the castle and included a massive lake, making it very difficult for attackers to get close to the castle. The siege lasted nearly nine months before disease and hunger finally defeated the defenders.

With England at peace, King Henry III acted to bring peace with Wales and signed an accord with Llywelyn ap Gruffudd in 1267, confirming Llywelyn and his heirs the title of 'prince of wales', as well as fealty and homages of 'all the Welsh barons of Wales'. This fuelled the dispute between Llywelyn and de Clare over the control of upland Glamorgan, and caused de Clare to start work on a castle at Caerphilly to protect his interests in the area.

Construction began on a massive castle built to a concentric design with successive lines of defence set one inside the other and all surrounded by a series of large water defences.
Llewelyn saw this move as a direct threat to his authority and in the autumn of 1270 he attacked and burnt the rising Caerphilly fortifications. De Clare immediately began building again.

In August 1274, the new king, Edward I, returned to England from crusade. Between December 1274 and April 1276, Llywelyn failed to respond to five summonses to do homage to King Edward as his Lord. In November 1276, Edward reacted by embarking on all out war with Llywelyn. Within twelve months Llywelyn had been driven from the Marches and much of the rest of Wales, and in a second royal campaign six years later, Llywelyn was killed and with him any hopes of an independent Wales.

With the threat from Llywelyn removed, Caerphilly castle was used less as a fortress and more as a centre for control and administration of the de Clare estates in the Glamorgan uplands. The Red Earl died in 1295 and the estates passed to his son Gilbert. Gilbert was killed at the battle of Bannockburn in 1314, ending the de Clare male line.

The de Clare lands came under Royal control while their future was decided. During this time, the Royal custodians provoked a Welsh revolt in the area which led to Llywelyn Bren leading an attack with 10,000 men on Caerphilly Castle in January 1316. The attack failed with virtually no damage sustained to the castle, and Llywelyn was later captured and imprisoned in the Tower of London.

Eventually, in 1317, the de Clare inheritance was partitioned between Earl Gilbert's three younger sisters. Eleanor de Clare (d. 1337), the eldest of the late Earl Gilbert the Red's three daughters, was married to Hugh Despenser the younger (d. 1326). A favourite of King Edward II, he held the court position of chamberlain. Able to pick which part of the de Clare estates he wanted he chose the Lordship of Glamorgan and Cardiff. But Despenser was a greedy and unscrupulous man, and not content with just this share, he soon set out to grab the de Clare inheritance from his brothers-in-law, Roger Damory (d. 1322) and Hugh Audley (d. 1347).

In 1318, Despenser had Llywelyn Bren brought to Cardiff where he was hanged, beheaded and quartered. It was just another act that led to a universal hatred of Despenser in south Wales. In May 1321 a powerful group of Marcher Lords rose against him, burning and destroying his property. The barons used this opportunity to enforce their complaints with the King, and Despenser and his father were banished. But before the end of the year, the Despensers were back in England, and with the support of the King, the barons were defeated at the battle of Boroughbridge in March 1322.

Such was his influence over the King, during the next few years, Hugh Despenser was the effective ruler of England. In South Wales he continued the expansion of his lands until he controlled most of the land between Chepstow and Pembroke. During this time he built the great hall at Caerphilly Castle.

But there was soon a new threat to Despenser and King Edward II. In September 1326 Roger Mortimer (d. 1330), a prominent member of the baronial opposition, and King Edward II's estranged wife queen Isabella (d. 1358), landed a small force from France. Edward and Hugh the younger left London and fled to the West, spending some time at Caerphilly Castle before moving on. They were eventually captured and Hugh Despenser was executed on 20 November, while King Edward II was forced to abdicate in favour of his young son.

From this time on the castle's role as a fortress and as the domestic residence of a Lord declined, with its various owners preferring the more comfortable surroundings of other properties. The castle fell into ruin, with some of its stone being removed and used as building material elsewhere.

In the late 19th century there was interest in protecting the castle from further ruin, with the great hall reroofed in the 1870's. Between 1928 and 1939, John Crichton Stuart, the fourth marquess of Bute, undertook the restoration of Caerphilly Castle, rebuilding many of the collapsed buildings and walls and relandscaping the defences. When the castle was taken into State care in 1950, the work continued. The reflooding of the lakes was completed and the great hall was restored and its windows reglazed.