The slate reference is a size of roofing slate, apparently sizes run through royal names starting with king. In the A clue there are 2 links to this pop singer both with the same answer, one female worker gives the 2 part answer and the castle reference gives the same in 1 word (in a G&S Savoy opera I think). The shepherd is a little obscure, as mentioned previously on the thread, and it's not a great definition for something they would carry to treat infections I think.