the Present Plot against his Majesty and Government,’ London, 1679, 4to.
Oates having meanwhile fortified his case by the fabrication of fresh evidence, Castlemaine was examined before the king in council, and re-committed to the Tower on suspicion of complicity in the so-called Meal-tub plot on 2 Nov. 1679. He remained a close prisoner until his trial before Lord-chief-justice Scroggs at the king's bench on 23 June 1680. The crown was represented by Attorney Sir Creswell Levinz [q. v.] Solicitor-general Sir Heneage Finch [see Finch, Heneage, first Earl of Aylesford], Sir George Jeffreys [see Jeffreys, George, first Baron Jefferys], solicitor-general to the Duke of York, and Sir Francis Wythens [q. v.] Castlemaine defended himself, and with such signal skill and courage that, though much interrupted and browbeaten by court and counsel, he completely discredited the evidence of the informers and secured an acquittal.
Castlemaine was a member of the little cabal of catholics who formed James II's secret council; and when the king determined to establish overt relations with Rome, Castlemaine was accredited ambassador to the curia. He embarked at Greenwich on 15 Feb. 1685–6, and reached Rome on Easter-eve (13 April, N.S.), but, though privately received by the pope (Innocent XI), did not enter the city in state until 8 Jan. 1687 (N.S.). The delay was owing partly to Innocent's illness, and partly to the elaborate preparations which Castlemaine thought it necessary to make in order to sustain his master's dignity. His major-domo, John Michael Wright, has left a curious account of his pompous entry, and the cold reception accorded him by the pope (cf. list of authorities infra, and the satirical ode upon the embassy in Poems on Affairs of State, 1716, ii. 402). Castlemaine's instructions were to solicit a cardinal's hat for the queen-consort's uncle, Prince Rinaldo d'Este; a bishopric in partibus for the king's most trusted adviser, the jesuit Edward Petre [q. v.]; and to attempt the reconciliation of Innocent with Louis XIV. He found Innocent by no means propitious. He had no intention of being reconciled to the author of the Gallican schism as long as the Gallican schism continued; he had little faith in the stability of James's throne, and less in the policy of attempting the forcible conversion of England. With much ado, Castlemaine induced him to confer the coveted hat on Prince Rinaldo, 2 Sept. 1686. In regard to Petre, his holiness proved inexorable. Not content with a first or even a second refusal, Castlemaine pressed his suit with more zeal than discretion in several audiences, which Innocent terminated by violent fits of coughing. Irritated by this treatment, Castlemaine at last sent him a written memorial not obscurely hinting at his possible departure if it were to continue. Innocent replied drily that he was his own master, and added significantly that the morning hours—it was summer—were best for travelling in Italy. Castlemaine remained, however, until, at Innocent's instance, he was recalled by James, who humbly apologised for his agent's excessive zeal. On 16 June 1687 Sunderland, as president of the privy council, was compelled to write to the pope, begging pardon for the ambassador's misbehaviour (cf. abstract of correspondence between the English court and the pope in Dod's Church History, iii. 424–5).
Castlemaine reached London in August 1687, and was consoled with a place in the privy council, being dispensed from the oaths, and with bounties to the amount of between 1,800l. and 2,000l. His name appears among the signatures to the certificate of the birth of the Prince of Wales, dated Whitehall, 10 June 1688 (Addit. MS. 27448, f. 342). On the subsequent flight of the king, Castlemaine left Whitehall for his country seat in Montgomeryshire, taking with him, under a privy seal, plate from the royal household, for which damages were afterwards (22 May 1691) recovered against him, to the value of 2,500l., the privy seal being held invalid by reason of its being subsequent to the ‘abdication.’ He was arrested at Oswestry, sent back to London, and committed to the Tower in February 1688–9, for ‘suspicion of treasonable practices.’ On 28 Oct. 1689 he was brought to the bar of the House of Commons, and examined touching his embassy to Rome. He pleaded in justification the express command of the king, but was recommitted to the Tower on the capital charge of ‘endeavouring to reconcile this kingdom to the see of Rome,’ and ‘other’ (unspecified) ‘high crimes and misdemeanours.’ On 10 Feb. 1689–90 he was released, giving his own recognisance in 10,000l., and those of four sureties in 5,000l. each. He was excepted from the act of indemnity, and was recommitted to the Tower in the following August on suspicion of complicity in the Jacobite plot, but was released on bail on 28 Nov. In 1695, having been for some years abroad in France and Flanders, he fell under suspicion of adhering to the king's enemies, was summoned to attend the Irish parliament on 12 Sept., and, failing so to do, was indicted of high treason. To avoid outlawry he returned to England, surrendered himself
