The castle is sited on a strategic rocky promontory by Pembroke River. 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 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 storeys. 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.
In the late 13th Century, more buildings were added. These included a new Great Hall that enclosed a large cavern beneath the castle that was created by water erosion. The cave, once fortified, may have later served as a boathouse.
The outer ward was defended by a large gatehouse, barbican and several massive round towers. The outer wall is 5 metres (16 ft) thick in places.
Pembroke Castle superficially resembles a concentric castle but it is more accurately described as a linear fortification because, like the later 13th-century castles at Caernarfon and Conwy, it was built on a rock promontory surrounded by water. This meant that attacking forces could only assault a narrow front. Architecturally, Pembroke’s thickest walls and towers are all concentrated on its landward side facing the town, the river creating a natural defense around the rest of its perimeter.
Extract from Wikipedia