Iambic Tetrameter
Alright let's dig a little deeper into the nuts and bolts of this poem. Most, though not all, of the poem is written in a pattern called "iambic tetrameter." This means that each line contains four ("tetra-" means four) iambs, as you can see in line 1:
And did those feet in Ancient time
Alongside this mostly regular meter (there are a few small exceptions that we'll get to in just a second here), there is also a mostly regular rhyme scheme of ABCB, where each letter stands for a particular end rhyme. This means that, in each stanza (except the third, which we'll also get to in just a second), the second and fourth lines rhyme, and the other two do not.
Now let's get to those exceptions, which are mostly confined to the third stanza (that's the one with all the weapons in it). The speaker switches both the meter and the rhyme scheme. Instead of using all iambs, the speaker uses two spondees for the first two beats and two iambs to finish out the lines. We give you line 9:
Bring me my bow of burning gold
There's a whole lot of stress in this line, and by stress we mean emphasis—not the "I have a paper due to tomorrow, and I haven't started" kind of thing. The "heaviness" of the lines calls attention to them, and makes them sound louder, stronger, and more aggressive, almost as if they were part of a rallying cry to troops in battle. The meter complements the speaker's sudden resolution to commit himself to the task of building Jerusalem again in England.
As for the rhyme scheme, this stanza rhymes ABAB. The stanza is thus more compact. Each line has a partner, which makes the whole thing seem more "whole." If you compare this stanza to the other stanzas that rhyme ABCB, it feels like those other one have some loose ends (the A and C lines). The wholeness or completeness of the third stanza mimics the speaker's desire for there to be another Jerusalem, which is the definition of a complete, whole, peaceful, and holy city where all the divisions and tensions of the world are eliminated.
The sudden shift in the third stanza is part of the speaker's larger desire for change. He talks about how maybe, just maybe, Jerusalem was once built in England before, but now all trace of it has vanished and there are only "dark Satanic mills." The speaker wants to get rid of these mills. He wants change. The poem's formal changes (in meter and rhyme) reflect in miniature the speaker's desire for that change.