
Do seek it out, but for those who haven't the time, here is the text. (I trust that SMBC won't mind me quoting it.)
You are standing in Castle yard, so called because it is believed to be the site of the town's castle as referred to in a chronicle of the reign of Henry II.

The stone walls of the castle were demolished by the late 1770's and the site was levelled in the 1770's in preparation for the construction of Castle Mill.
Subsequent levelling was for the demolition of the mill in 1841 and the creation of a cattle market in 1853.
The design of the mill was unusual in that it was round. Most mills were rectangular. The design looks to have been influenced by being on the site of the castle.

The wheelpit was revealed during the recent excavation works. It shows a large rock cut feature containing various rock cut niches, slots, basal channels and two tunnel openings that appear to have been structural components of the 18C water wheel housing and water flow control system.

PUBLIC ART Created by Mike Woods
The artist was asked to base the design on a water wheel, following the discovery of the original wheel pit.
Mike explains that the work used, as a starting point, the water wheel which once powered Castle Mill. He goes on to explain:

For more details on:
the water tunnels under Stockport;
the 18C battles for water supply;
the Trust's role in locating and identifying the waterwheel pit; and
how the pit and a medieval well now lie beneath TK Maxx - then talk to Underground Stockport researchers, such as: Kevin, Coral, Allan, Jim or Steve.
Click here for a Stockport Express article which tells some of the story.