Horace Walpole (1717–97) is well known for two important Gothic projects: his villa, Strawberry Hill, Twickenham (1747/8–80), and his novel, The Castle of Otranto (1764). These two manifestations of Walpole’s ‘Gothic imagination’ are frequently linked in critical literature on the Gothic Revival and medievalism more broadly; the relationship between Strawberry Hill, Otranto and manuscript illustrations visualising Otranto’s narrative has, on the other hand, received far less attention. This paper brings together a number of important and hitherto overlooked sources that help address this imbalance. In particular, it examines two large-scale watercolours by John Carter (1748–1817) that narrate some of Otranto’s pivotal scenes, allowing critically overlooked subtleties in their iconographies to emerge. The work establishes how Carter’s pre-existing interests – in particular, in Gothic architectural forms and heraldry – are harnessed to govern his representations of Otranto. These paintings, together with Carter’s other illustrations, demonstrate Walpole’s authorship of Otranto, expressed through codes hidden in plain sight. Unlike the frequently touted link between Strawberry Hill and Otranto in secondary criticism, Carter’s illustrations, the argument reveals, do not explicitly make this connection.