Pembroke's strategic importance soon increased, as it was from here that the Normans embarked upon their Irish campaigns. 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 original entrance was on the first floor, approached by an external stair, the present ground-floor entrance being a later insertion. The keep had four floors, connected by a spiral stair which also led to the battlements. The large square holes on the top of the outside were to hold a timber hoard, or fighting platform. When the castle was attacked, the hoard could be erected as an extra defence, outside the battlements but way above the heads of the attackers.
In the late 13th century, additional buildings were added to the inner ward including a new Great Hall. A 55-step spiral stairwell was also created that led down to a large limestone cave, known as Wogan Cavern, beneath the castle. The cave, which was created by natural water erosion, was fortified with a wall, barred gateway and arrowslits. It may have served as a boathouse or a sallyport to the river where cargo or people could have been transferred.
Although Pembroke Castle is a Norman-style enclosure castle with Great Keep, it can be more accurately described as a linear fortification because it was built on a rock promontory surrounded by water. 
This meant that attacking forces could only assault a narrow front. 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.
This castle is what I always envisioned a castle should be. The layout is complex and there are multitudinous passages, stairways, towers and halls offering endless scenarios. It literally took me 2 hours to walk around this castle, and I still didn't see it all. The spiral staircases are treacherous and the towers are precipitous. 
There was even a real dungeon (with a stuffed prisoner..)! This would be the most awesome place for kids to play, albeit a bit dangerous. Even I felt like playing hide and seek or capture the flag here, and I am pushing 40 (going on 10).
