In A Mask Presented at Ludlow Castle (1634), John Milton depicts Comus “ripe and frolic of his full grown age, Roving the Celtic and Iberian fields.” While Milton’s complex engagement with Portugal and Spain has been the subject of some discussion by critics, few attempts have been made to place his writings on the Iberian Peninsula within the wider context of his theories of climatic influence and colonialism, beyond the “western design” against Spanish colonial possessions. Anti-Catholicism and anti-imperialism may be the key to Milton’s Cromwellian correspond-ence with Spain and Portugal on behalf of the English republic in the 1650s but his Iberian interests can be viewed too as part of a deeper excavation of British and Irish histories. The purpose of this article – its “roving commission” – is to explore the presence of the Peninsula in Milton’s work from “Lycidas” (1637) through to The History of Britain (1670) in relation to recent archipelagic readings of Milton, examining the ways in which Celtic and Iberian concerns are intertwined in Milton (as indeed they were for his predecessor, Edmund Spenser).