"Gretel in Darkness" is as dense with alliteration as a forest with trees. In this poem of terror and trauma, that alliteration helps to evoke Gretel's claustrophobia. Walls of matched sounds spring up around her, trapping her in her terrible memories.
The alliteration here often points out associations between words, drawing Gretel's mind helplessly back to her horrific experience in the witch's cottage. In the third stanza, for instance, alliteration on the /k/ sounds of "killed" and "kiln" lead Gretel back to the fact, never directly stated in the poem, that she killed the witch by burning her alive in her own oven. (See the entry on assonance for more on these linked words.)
Similarly, the /f/ sounds in "far from," "father," and "forget" in the second stanza emphasize Gretel's dilemma: though her father wants her to believe that she's far from all harm, she still can't forget what happened, and the alliterative link between these words helps to suggest Gretel's difficulty in fitting her father's reality together with her own deeply-felt experience of the world.
Back in the first and second stanzas, alliteration on /w/ sounds draws a sinister connection between "women" and "witch"—a connection that ties Gretel herself into a legacy of female murderousness. Those /w/ sounds also spring up densely in the very first lines, so that the "world we wanted" and "all who would have seen us dead" are linked to these dangerous witch-women. The linkage of sounds here supports Gretel's feeling that, though the external danger is apparently gone, the internal danger is as present and real as ever. Escape, it turns out, isn't so easy.