The ridge of land on which the town of Pembroke now stands had on the north and south side, a tidal inlet extending a mile and a half eastwards of the present castle ruins. The western end of this peninsula was strengthened be defence works, which comprised of three ditches and vellums (earthen ramparts) across it from share to shore. Across the southern tidal inlet on the land now occupied by part of Monkton, was a larger earthwork. Inside this Monkton earthwork, a church and some monastic buildings were established in the early centuries of the Christian era. Just how long these buildings existed before the arrival of the Normans is impossible to any but about 1090 AD Arnulph de Montgomery came and occupied this Monkton earthwork. The Welsh are said to have attacked in 1092 AD and again in 1094 AD but without any success, so the earthworks must have been a very strong defence. In 1098 Arnulph de Montgomery was so well established in the district that he made a gift of the Church of St Nicholas 'within his castle' to the Abbey of Seez in Normandy. The North side of the nave of the present church, supported outside by buttresses, is the original wall of the ancient Church of St Nicholas. The last Prior was William Warren, who ruled in 1534.
In 1456 Henry VI I was born in Pembroke Castle and as a boy he went to the Priory to be brought up and educated by the Benedictine Monks. He remained here until his teens. When Oliver Cromwell besieged Pembroke Castle in 1648, he placed a battery of cannon at Monkton, some say in the paddock alongside the present vicarage. The then curate in charge refused to leave his home and preferred to risk the dangers and noise. After the siege he was invited by Cromwell to preach to the Officers of his army.
After the Dissolution of the Monasteries under Henry VIII, the condition of the Priory Church gradually deteriorated until there remained only the long narrow nave, which was the old place of worship for the parishioners. The choir and sanctuary became a roofless ruin. The large west window was lost and the beautiful geometric windows on the south side were blocked up (two still remain blocked). The arch where the lectern and pulpit now stand was bricked up and the rich canopied tomb of Purbeck marble (early le century) was placed against the wall. This tomb is now in the north wall of the sanctuary. It was in this condition and worse that the Rev David Bowen found his church when he came to be the Vicar of Monkton in 1877. He gave his life to the work of restoration and was Vicar of this Parish for nearly fifty years until 1926. A tablet erected to his memory on the North wall of the Chancel has these Latin words: 'Si quaeris monumentum circumspice' -'If you are looking for his memorial, look around you.'
The Church today in restoration, the floor level of the porch was lowered a couple of feet, and in doing this the large stone slab, which probably for centuries had done duty as a door step, on being turned over proved to be the effigy of someone, no doubt a former Prior. This newly discovered effigy is now seen in the South wall of the Chancel, whilst in the North wall recess in the Sanctuary another effigy is to be seen, being that of a Crusader or of a patron of the Church. Over the present Porch there used to be a room called a `Parvisse'. The steps which led to this room can still be seen to the West of the Main Door, running inside the wall. When this `Parvisse' was opened at the restoration, a skeleton of a monk was found in such a position to suggest that he had been locked in alive. A very One Norman arch covers the doorway to the Church and this too was only discovered at the restoration.The long, narrow Nave has a vaulted plaster roof which, because of its height and length, is probably a unique example of this craft in the country. The North side is double walled and is supported outside by three huge buttresses. This side is the oldest part of the Church built there in the very earliest centuries AD.
When the floor of the Nave was levelled, hundreds of human bones were found, mingled in a most extraordinary fashion - skulls, legs, ribs and other portions of human remains resting side by side or crossing each other, without any order or arrangement of any kind. These were reverently collected and re-interred in a large grave 12 ft square and 7 ft deep under the shadow of the North Wall of the Churchyard. The small Recess in the wall near the Porch door was probably for a vessel containing Holy water.
The vaults of the Meyricks of Bush, Owens of Orielton and Corston Family are still legible. The figures of the seven sons and a daughter of the Meyrick family are to be seen kneeling in perpetual prayer for their father's soul around his vault. In 1887 the wall dividing the Nave from the Choir was removed and in its place the present Chancel Arch was built, with its cluster of columns surmounted by the heads of St Nicholas and a monk. The arch is 45 ft high and almost as wide as the full width of the Nave (24 ft).
The Chancel and the Sanctuary, formerly the Benedictine Choir, are on an unusually large scale, 69 ft high. The great East Window was created to commemorate the visit in 1902 of King Edward V11 and Queen Alexandra. It was dedicated in 1904 by the then Bishop of St Davids. The Canopies over the Choir Stalls were designed by Mr C E Halliday of Llandaff, and were carved by Mr Edwin Thomas of St Davids. On the North Side of the Chancel there is a glass case which contains a chained Book of Common Prayer which is very rare. It was recovered in 1904 and at Evensong on Whitsunday of that year it was replaced in the Chancel. It is the 1604 Prayer Book which Cromwell suppressed when he introduced the 'Directory' into the Church of England. Also in the glass case are a Monk's beer jug and a Holy Water stoup. To the north of the Chancel is a doorway which leads to a large Chapel which in all probability was the Prior's private chapel. This too was in ruins and has been restored.
The Vestry and Organ Chamber were at one time 'barrel' shaped passages to the Monks' domestic buildings and there are still signs of the upper floor which was the Infirmary. One can still see a large opening in the upper wall into the Chancel, through which sick monks could see the High Altar and the Elevation of the Host during Mass.
