Assonance, consonance and alliteration are all literary devices based upon sound, and used most often in poetry—they are what gives poetry a "musical, lilting" feel. All of these devices are based upon the repetition of the same sound, created by vowels (in assonance, primarily) and consonants (in alliteration and consonance). It does not matter if the letters are the same: "ph" and "f" have the same sound. "Leaves" and "meet" have the same "e" sound though the vowels are different.
Assonance is the repetition of a vowel sound found in a group of words. An example would be: I'll make a date with fate. The long "a" sound in "make," "date" and "fate" create assonance.
Assonance may be found in lines ten and eleven with...
To please the yelping dogs. The gaps I mean,
No one has seen...
The long "e" sound in "please," and "mean," and "seen" (found in the next line) have the same sound. They do not have to be on the same line, but need to create to one's ear a clear, repetitive pattern of the same sound.
We "hear" it again in line twelve:
But at spring mending-time we find them there.
The repetition of the same sound is found with the long "i" sound in "-time" and "find."
Your example of alliteration with "Before" and "built" are perfect, repeating the same sound at the start of words generally clustered together (as is this case with the "b" in "Before" and "built"). It is also seen in line 40 with "stone savage" (with the repetition of the "s" sound).
Consonance is, of course, the repetition of the same sound using consonants. We can find examples in Frost's "Mending Wall" in line 13:
And on a day we meet to walk the line...
Consonance occurs in the words "And," "on" and "line" with the repetition of the "n" sound. In the following line, we "hear" consonance again with two different examples:
And set the wall between us once again.
The repetition of the "t" sound is found in "set" and "between," and again with the "n" sound in "And," "between," "once," and "again."
Whenever you search for these devices, it is best to read the lines aloud and let your ears do the work.