This poem is a slow read, notably because of the lack of enjambment. If you examine the ends of the lines, you'll find a hard stop at almost every line break. Additionally, the poem contains significant use of caesura, which is an intentional pause within a line itself. Consider all the times the punctuation slows down the reader's progress just in this section:
'Twas so; but this, all pleasures fancies be.
If ever any beauty I did see,
Which I desired, and got, 'twas but a dream of thee. (Punctuation bolded for emphasis)
These intentional efforts to pause and go slowly reflect the tone of the speaker. He wants to convince his beloved that the love they share is of paramount significance to him. In fact, he believes that they are "two ... hemispheres," forming a complete circle only when they are joined.
The speaker also employs rhetorical questioning, asking "I wonder, by my troth, what thou and I / Did, till we loved?" Of course, he really isn't expecting an answer here. The question reinforces the great strength of his love; he can't even recall a life before this woman.
Alliteration is used throughout the poem. One example is here:
Or snorted we in the Seven Sleepers’ den?
The repetition of the s sound here is reminiscent of the whispers of lovers as they lie in bed, awaiting sleep. This is also an example of allusion, a reference to a Catholic tale about a group of Christian children who were walled up alive by their emperor; instead of dying, they were supposedly found alive and well nearly two hundred years later. This example of allusion contributes to the mystical feeling the speaker attributes to the love he shares with his beloved.
The poem also uses an example of chiasmus, which is when grammatical constructs are repeated in reverse order:
My face in thine eye, thine in mine ...
This structure mirrors the reflective nature of the two lovers as they gaze into each other's eyes.
Together, these devices contribute to a highly intimate tone that praises the incredible love between the speaker and his beloved.