Where It All Goes Down
It's a lovely night and we've joined our speaker for a bit of peaceful reverie. The sun has gone down and the night sky is full of dim stars. The green moss below us (and several clues from the speaker) indicate that it's springtime. There's a castle, a nearby grove, and a body of water below us.
And… well, those are all the clues we get. Though it's widely believed that Coleridge was addressing Wordsworth, his fellow British Romantic (which may help make sense of the castle), this much else is not overtly indicated in the actual poem.
This, when you think about the topic, might be on purpose.
After all, Coleridge was talking about big, universal ideas like poetry and nature. He wanted people to sit outside and examine the beauty of nature.
That's a pretty timeless message, so perhaps he didn't want to date it by making the era clear.