One, if by land, and two, if by sea (line 10)
This might seem kind of obvious at first, but we think it's worth thinking about: this famous line is all about nature. The poem is concerned with an event, and human actions, but it's full of geography and the landscape, too. We hear a lot about rivers and tides and fields and trees. It's easy to overlook, but the land and the sea are a big part of what makes this poem fit together so well.
Just as the moon rose over the bay (line 17)
Here's another big nature moment, partly because the moon is such an important part of the world of the poem. The moon and its light reminds us that the ride is happening in secret, in the dark, but it also lends a spooky mood to much of the poem.
Where the river widens to meet the bay (line 54)
The speaker gives us a really specific detail here. This isn't just some generic river; it's a specific spot on the Charles River in Boston. We are supposed to get a feel for that specific spot and what it would be like to look down on it from the church tower on an April night. The poem is about big-picture issues in American history, but it shows them through careful and interesting, images like this one. The flow of water and the New England landscape both help keep this poem grounded in the real world.