I just retagged all 6 castle-project questions to castle. I didn't have enough upvotes in either tag, so I couldn't suggest a synonym. Since it weren't that many questions and I'm confident that this is a correct change, I retagged manually.
The tag will remove itself within 24 hours. Tags with 0 questions are always removed (except for the preloaded tags on metas) within 24 hours, unless you sneak that tag onto a question in the meanwhile.
Synonyms are more for when the tag keeps getting created. I doubt that this tag will keep getting created, someone must have accidentally created it at first and then people used it because it came up in autocomplete.
So no, no need to take any action :)