The earliest confirmed occupation of the site of Westminster was a Benedictine Abbey set up by St. Dunstan in the 960s. The beginnings of the Abbey as we know now are from the reign of King Edward the Confessor (r. 1042-1066). On the site he built a church dedicated to St. Peter and a monastery in the Romanesque style. The church was consecrated shortly before King Edward's death.
Henry III (r. 1216-1272) decided that he wanted to replace Edward's abbey with a grander structure. The original building was demolished and rebuilt progressively from east to west. None of Edward the Confessor's old church survives, but parts of the Norman monastic buildings around the cloister remain today.
The new Gothic church was created by Henry de Reyns and was taller, lighter and more spacious then the old Norman building. The new building also used better stone -- sandstone from Reigate, limestone from Caen (in Normandy) and polished Purbeck stone from Dorset for pillars.
The style of the church had origins in French cathedral building: the basic layout of the apse with radiating chapels, large windows and wall arcades. Also from the French style are the rose windows and flying buttresses. This style created a high roof which reaches 103 feet. The cathedral also has English elements, such as the long nave and broad transepts. The decoration with sculpture and elaborate arches, as well as the polished Purbeck stone are English qualities as well.
The reconstruction of Westminster Abbey that had begun in the reign of Henry III in the 13th century was finally completed when the nave was finished in 1517 during the reign of Henry VIII. The monastery that had been on the site since the reign of Edward the Confessor was surrendered in 1540 during the dissolution of the monasteries in the Reformation.
From 1540 to 1550, the building became the cathedral for the new diocese of Westminster. However, when Mary I became Queen and brought the Catholic faith back to England, once again a community of monks took up residence at Westminster, but it was a brief tenure. When Mary died and Elizabeth I became Queen, Westminster became a collegiate church with a dean and a chapter of 12 canons.
All of the crowned Tudor monarchs except Henry VIII are buried in the Abbey (Henry is buried in St. George's at Windsor Castle). Henry VII shares a tomb with his wife Elizabeth of York. His mother Margaret Beaufort is also buried nearby. Only one of Henry VIII's wives is buried in the Abbey - Anne of Cleves. Edward VI, Mary I and Elizabeth I are also interred there, with Mary and Elizabeth sharing a fine monument constructed by James VI/I. After James became king, he had his mother Mary Queen of Scots re-interred in a splendid tomb at the Abbey next to his paternal grandmother Margaret, Countess of Lennox.
The west entrance to the abbey didn't get its iconic tall white towers until the18th century.