Hi Scarlett. I can't help you with the genealogy, but did add the Casteel surname to the Steele Surname project. It was already included in the Castle Surname Project. Here are the links to the two projects:http://www.familytreedna.com/DNAList.asp?Group=Steelehttp://www.familytreedna.com/DNAList.asp?Group=Castle
You can test a male Casteel (Steel) member of your family and have him joined to both projects (after the results are back) as you search for your genetic kin and genetic ancestry. If he was Cherokee from his father's fathers' ... father's paternal line - that will also be shown in the yDNA results.
It may also be useful to compare your yDNA results with the Sparks family - if the results are not Native American