ly Cathedral's site dates back to at least AD 673, when a monastery for both monks and nuns was founded by St. Etheldreda. Etheldreda died on 23 June 679 and on 17 October 695 her body was moved from the monastery into the Saxon church. On the same date in 1107 and again in 1253, her body was moved into improved locations. The dates of 23 June and 17 October are now important festival dates in the Cathedral's calendar. The area of the fens on which the Cathedral stands would have been more water logged leaving the Cathedral sitting on an island. The Danes attacked the monastery in 869 and left the buildings totally destroyed. In the reign of Edgar and under the direction of Dunstan, the Archbishop of Canterbury and Ethelwold, Bishop of Winchester a monastery at Ely was reinstated. The first Norman abbot, Simeon started rebuilding work on parts of the existing building around 1083, a couple of years after he was appointed. Ely was then given the status of a bishopric in 1109. The west front of the Cathedral is odd because the northern side of it collapsed in the fifteenth century leaving the remainder lopsided. When the central tower collapsed in 1322 a new design was incorporated resulting in the unique octagonal central tower and lantern.
Mini Timeline

1083		Ely Cathedral building begins
		Work began on Ely Cathedral
1106	Oct 17	St. Etheldreda's Day
		In 1106 or 1107 at Ely Cathedral the remains of saint Etheldreda were moved into the newly built presbytery. This was done on the saint's day, October 17th.1
1109		Church at Ely becomes seat of bishop
		The church at Ely became a Cathedral when it became the seat of the bishopric (or Diocese) of the area.
1252		Shrine to St. Ethelreda installed at Ely
		A shrine to St. Ethelreda was installed in the retrochoir of the cathedral. The ceremony was attended by Henry III and many other dignitaries.
1321		Lady Chapel construction at Ely
		The construction of the Lady Chapel (also known as the Trinity Chapel) was started in 1321 at Ely Cathedral. Shortly after work began the central tower fell down damaging the choir area.1
1322		Collapse of central tower at Ely Cathedral
		The collapse of the central crossing tower of Ely Cathedral allowed the construction of the unique Octagon that took its place.