In 1093 Arnulf of Montgomery built the first castle at the site when he fortified the promontory beside the Pembroke River during the Norman invasion of Wales. A century later, the castle was given by Richard I to William Marshal, who became one of the most powerful men in 12th-century Britain. He rebuilt Pembroke in stone creating most of the structure that remains today.
The castle is open to the public and is the largest privately owned castle in Wales.
The castle is sited on a strategic rocky promontory by the Milford Haven, and the Cleddau Estuary. The first fortification on the site was a Norman motte-and-bailey. It had earthen ramparts and a timber palisade.
In 1189, Pembroke Castle was acquired by William Marshal. The Earl Marshal then set about turning the earth and wood fort into an impressive Norman stone castle. The inner ward, which was constructed first, contains the huge round keep with its domed roof. Its original first-floor entrance was through an external stairwell. Inside, a spiral stairwell connected its four stories. The keep's domed roof also has several putlog holes that supported a wooden fighting-platform. If the castle was attacked, the hoarding allowed defenders to go out beyond the keep's massive walls above the heads of the attackers.
The inner ward's curtain wall had a large horseshoe-shaped gateway. But only a thin wall was required along the promontory. This section of wall has a small observation turret and a square stone platform. Domestic buildings including William Marshal's Great Hall and private apartments were within the inner ward. The 13th Century keep is 23 metres (75 ft) tall with walls up to 6 metres (20 ft) thick at its base.