Of Reformation in England
P. 52. their damned designs: the restoration of Papacy and ecclesiastical despotism.
P. 53. antichristian thraldom: he would seem to allude to the invasions of England by the Romans, Saxons, Danes (twice), and Normans, and the War of the Roses, followed by the partial reformation under Henry VIII.—Keightley.
P. 53. Thule: some undetermined island or other land, regarded as the northernmost part of the earth; called in Latin Ultima Thule; often used metaphorically for an extreme limit.
P. 53. that horrible and damned blast: Keightley understands this as referring to the Gunpowder plot.
P. 53. that sad intelligencing tyrant: Philip IV., King of Spain from 1621 to 1665.
P. 53. mines of Ophir: used in a general sense for gold mines.
P. 53. his naval ruins: an allusion to the destruction of the Spanish armada, in 1588, in the reign of his grandfather, Philip II.
P. 54. in this land: when Milton wrote this, he must still have been meditating a poem to be based on British history.