Assonance is used throughout the poem, but is particularly evocative in line 8, line 10, and line 12.
In line 8, the /o/ sounds of "so," "flowers," and "bore" are not exactly the same, yet are close enough to create a flowing and open vowel sound that contrasts with the harsher /t/ sounds that fill lines 5 and 6 (and which have signified the gates/chapel being closed off). These /o/ sounds are the very opposite of closed off, instead suggesting openness—even creating openness in the movement of the mouth as the lines are read.
To be fair, the phrase "over the door" creates a similar sensation of openness in line 6, yet this follows directly on the heels of the sharply consonant "thou shalt not." The assonance here could be said to make that instruction sound all the more restrictive by contrast.
The /o/ sound is again used in line 10 to link "tomb-stones" with "flowers," but to different effect. Here the /o/ sound feels heavy and cumbersome, suggesting the weightiness and immovability of the "tomb-stones" as they overtake the "flowers." Indeed, the fact that the phrase uses the same sound as line 8 helps to show the way in which death and negativity have taken over the Garden of Love—they are now even taking over its sounds!
In line 12, "binding" and "briars" link together through the long /i/ sound with "my" and "desires," highlighting that it is these personal desires themselves that are no longer allowed to flourish (i.e., that are bound by the priests).