There are other, more famous, wells in Glastonbury, but I find "St Joseph's Well", in the Abbey, quiet, pleasant and enigmatic.

It was certainly constructed before the Norman conquest. There is arguably evidence for its dating back to Roman times, although it's more likely that the pottery which is the most important dating evidence was actually imported as infill during the early mediæval period.

Be that as it may, mediæval or Roman, the well could easily have been the reason for the siting of the first church.

Originally the well-head was probably close to the current ground level, being truncated when the crypt of the Lady Chapel was built during the 1500s. However, the arch over the well looks suspiciously like the arch of a window from the former east wall of the chapel, demolished when the Lady Chapel and the Galilee were connected, which may have happened as early as 1350.

 
The entrance today is from the level of the crypt. Now that there is wooden flooring in place it's hard to get a good picture, but this section of an illustration from Frederick Bligh-Bond's book of 1909 shows the door more clearly (I've highlighted it). The other door, higher up on the left, is at ground level.

The gate is normally locked, so that throwing coins into the well for wishes turns into an elaborate game of pitch and toss. For those who plan ahead, however, it's possible to get it unlocked by prior arrangement with the custodian.

An earlier entrance route was from the chapel itself, down a set of steps through the outer wall which turn at the foot to face the well itself.

Bligh-Bond's book suggests that the earliest access was from the south, either via steps from the graveyard or by means of a tunnel, one of many, real or imagined, in the Glastonbury area. According to Bligh-Bond, the tunnel was still visible in about 1850, but was filled in shortly thereafter to prevent lambs falling into it...

 
The well was rediscovered in 1826 when the crypt of the Lady Chapel was cleared of earth. This is an illustration from Warner's book of 1826, showing the arch and well shortly after the crypt was cleared.

Although the well is known as St Joseph's well, this is probably because the crypt became St Joseph's chapel at the height of the cult of St Joseph of Arimathea in the 1500s. The church or chapel above it was the Lady Chapel much earlier than this.