Castelli was equally distinguished as a botanist, chemist, and surgeon. He maintained the necessity for all physicians of studying anatomy, and declared in 1648 that he had dissected more than one hundred corpses.
The DaneThomas Bartolinus (1616-1680) was led by Castelli's fame to visit him in Messina, in 1644, and speaks of his activity as a publicist. Castelli wrote no less than one hundred and fifty pamphlets. Among these there is one written in 1653 in answer to inquiries by Hieronymus Bardi of Genoa, wherein Castelli speaks of the cinchona plant and its curative properties in cases of malaria.
Paolo Boccone's pupil Charles Plumier (1646-1704) later perished on his way to South America to learn more of the cinchona.
Castelli seems to have had but little knowledge of the cinchona, and no experience in its medicinal application. Still, the pamphlet is noteworthy as being the first Italian publication that mentions the cinchona.