The well is located in the ditch at the foot of the Kings Tower Keep, and is 75 ft or 23m deep, in medieval measurements that is 20 ells. It was dug through the limestone very early in the castle's history. The depth is known because there is a record of a cord having been purchased for the well bucket during King Edward II's time.
Rosamunds Tower is the northernmost tower and stands astride the inner ditch, this allowed a small gate to be placed in the ditch bottom, (a postern). The postern had a small drawbridge and was built on the instruction of Edward II, but it is not known when or why the name Rosamund was given to the tower. 'Fair Rosamund' was the mistress of Henry II, but she had died more than a century earlier. The gate is also referred to as a Sallyport.