Could you please explain why the syntax in the following stanza is wrong?
by that sturdy assertiveness
that walled England the din
of traffic in my mind quietens,
There are two slightly different answers to your question. As a matter of formal grammar it is missing a comma.
However, there is something else worth pointing out. One of the things that sets poetry apart from prose is that it uses the form of words to convey meaning besides the strict semantics. This is true of prose to some extent, but it is a more important part of poetry. In simply poetry for example, rhyming schemes give a cadence to the meaning that enhances the reading pleasure.
In the example you gave the structure is rather awkward. The third line contains an obvious semantic break that isn't reflected well in the structure. I'd suggest this would be better:
You might even break the fourth line into two, as a kind of ragged cadence seems to be the author's intent.
I'd also say it is an interesting sentence. I think it is unusual the way there is a huge pile of words before you hit the verb. As you are reading all the qualifiers you are thinking, and thinking "what is the point of this sentence." I suppose it builds a sense of anticipation.
Can the OP share the source?
There is nothing I can see wrong in the syntax of that stanza. In fact, unlike much poetry, there is no serious lexical ambiguity; and with a single comma it would render just fine as a prose sentence:
Take "that sturdy assertiveness that walled England" to mean "the trait that formed a wall around England, which I will call 'sturdy assertiveness'" ... and thus shielded [as I am from such assertiveness], the noise of traffic in my mind becomes still.