Castro started training as a painter and printmaker. His illustrative and painterly style comes through in the design of his rings. His artwork is inspired by history, myths and biology - having always been obsessed with creatures and their creations.
The technique he uses, 'seal engraving' goes back to ancient times. A seal is always engraved in reverse and is much deeper than traditional two-dimensional hand engraving with a third or fourth dimension added which is deeper and finer and takes much more time to achieve. Traditionally the signet ring would bear the family crest and the ring would be stamped onto hot wax to seal important documents, with the Intaglio engraving acting as a signature, and the engraved image or crest reproduced in the wax as a 3D relief. Now, he uses his knowledge of engraving to create contemporary pieces and to experiment with European and Japanese engraving techniques, producing cross-fertilisation of styles and processes.