The "realms of gold" in the first line of the poem refers to the translation of Homer by George Chapman that Keats is reading. In this sonnet, Keats compares reading this translation to the discoveries the early explorers made as they circled the globe in search of physical realms (or cities) of gold. Specifically, he compares the thrill of first reading this translation to the awe Cortez and his sailors must have experienced when they first saw the Pacific Ocean. Keats writes that he felt like:
Stout Cortez when with eagle eyes
He star'd at the Pacific—and all his men
Look'd at each other with a wild surmise . . .
As critics have often pointed out, Balboa, not Cortez, discovered the Pacific, but the larger point is that Keats finds the discoveries he makes reading a great book as exciting as discovering new worlds or new oceans. The interior life of the imagination is for him as deep and full as any newly discovered place.