In this light and playful poem by Longfellow, the poetic speaker compares the children, "Grave Alice, and laughing Allegra, / And Edith with golden hair," in an implied metaphor, to an ambushing army storming a castle. He underscores this metaphor through an allusion to an old oral tradition folk tale called "The Mouse Tower," which features "the Bishop of Bingen / In his Mouse-Tower on the Rhine!" The Bishop was besieged by hungry town's people who repeatedly begged for wheat from his granaries to feed their children during a time of failed crops following the flooding of the Rhine River.
In this dark folktale (making it a surprising allusion for Longfellow's playful poem), the Bishop flatly refuses to aid them, which results in a tragedy that ends with his being ambushed by a vicious army of devouring rats. Longfellow inverts the intention of this devouring army when he says of the girls that "They devour [him] with kisses."
The words and phrases that sustain Longfellow's metaphor are many and liberally sprinkled throughout the poem. Some of these are: surprise, indicating ambush; raid, indicating army; castle wall, indicating assault. Others are:
plotting and planning
doors left unguarded
enter castle wall
climb up into my turret
O blue-eyed banditti
scaled the wall
fast in my fortress
down in the dungeon
Till the walls shall crumble to ruin.