I have studied the link and it looks very impressive however it is presumptive and there is no evidence whatsoever to suggest that any worthwhile remains of the castle exist.
Any one with any knowledge in the building trade will know that in the construction of a substantial building such as Castle Market then the foundations go down to solid ground ,ground that is below any previous construction.
This is what happened at Castle Market and during that construction no remains of any significance where uncovered.
Further to this I would like to point out that as champions of Sheffield's past history you ignore the trading area that has been on that spot before any castle was ever built and in so doing are ignoring an even more important part of Sheffield's history ,every day living history, that will be destroyed forever because of a minority group who have used their influence to erase a thousand years of Sheffielders trading on or near that spot.
Who knows one day we may have a thread on why did we destroy the Market area as we often still get in regard to the Rag and Tag.