Following the death of the Welsh king, Rhys ap Tewdwr, in 1093, the Norman baron, Roger de Montgomery invaded the deceased king's lands in south-west Wales and claimed them for himself. He made his base at Pembroke where he built a castle on a rocky promontory between the Pembroke River and the Monkton Pill. Roger's sons lost the castle to the crown following a failed rebellion, and in 1138 the earldom of Pembroke was created and given, along with the castle, to Gilbert de Clare. His son, Richard 'strongbow', used Pembroke as a base from which to launch an invasion of Ireland. On his death in 1176 he held the titles of Earl of Pembroke and Striguil, Lord of Leinster and Justiciar of Ireland. However he died without a male heir and the castle reverted to the crown, who continued to use it as a convenient base from which to monitor royal interests in Ireland.

In 1199, William Marshal was made Earl of Pembroke. He began to reconstruct the castle in stone, and started work on a large cylindrical tower or keep, the most impressive of its kind in Britain. From its splayed plinth the tower rises nearly 24m with a diameter of 16m and walls that are 5m at the thickest part. It is capped by a stone dome. The keep had four floors but these are now missing allowing you to stand in the basement and look up to the stone dome above. It is still possible to climb the spiral stairs to the top of the tower for spectacular views of the area. The tower offered little comfort and would only have been occupied as a last resort during an attack. At all other times the main accommodation was provided by a hall and other residential buildings built within the inner ward.

By the time all of Marshal's sons had died they had completed the inner ward and possibly the outer ward with its great towered curtain and large gatehouse, although this later project may have been the work of William de Valence who acquired the castle through his marriage to Marshal's grand-daughter, Joan. Following Valence's death the castle went into a period of decline under the ownership of a number of absentee landlords. This decline was temporarily halted when Jasper Tudor chose to make the castle his home. His nephew, the future King Henry VII, was born within the castle walls in 1457.

During the Civil War Pembroke was the only town in Wales to support Parliament. The mayor, John Poyer, garrisoned the castle in 1642 and strengthened the south wall with masonry taken from the Inner Curtain Wall. In 1648 Pembroke became the base for disaffected Parliamentary troops who declared for the king. Oliver Cromwell was sent to deal with the situation, and following a two month siege in which great damage was done to the castle by cannon-fire, the garrison surrendered. Cromwell ordered that Pembroke's defences should be demolished and each of the towers on the south front were destroyed by gunpowder.

In the 1880's, the Victorian antiquarian J.R.Cobb rebuilt the Barbican Gateway. In 1928 the ruins were purchased by Major-General Sir Ivor Philipps who began a ten year restoration of the castle. Today you can explore the rebuilt towers and walls, climb to the top of Marshal's circular keep and even descend a spiral staircase to the 'Wogan', a great natural cavern under the castle.