After a good preliminary education at Thame Grammar School, under William Burt (1608–1679), he was admitted a commoner of Balliol College, Oxford, on 8 April 1652, at the age of seventeen, and proceeded BA on 18 October 1654, MA on 29 May 1657.

Meanwhile he had gained a probationary fellowship at All Souls in 1655, and accumulating his degrees in physic proceeded MD as a member of that house on 21 June 1665.

In February 1669 he was elected a fellow of the Royal Society, and, as he himself indicates in the epistle dedicatory prefixed to his Chymical Galenist, had thoughts of presenting himself before the College of Physicians for examination as a candidate.

Afterwards, by the influence of his friend Martin Clifford, master of the Charterhouse, Castle was appointed physician to that institution, and obtained a respectable share of business.

[1] Castle was the author of The Chymical Galenist: a Treatise, wherein the Practise of the Ancients is reconcil'd to the new Discoveries in the Theory of Physick; shewing, That many of their Rules, Methods, and Medicins, are useful for the Curing of Diseases in this Age, and in the Northern parts of the World.