In this poem by Longfellow, "the children's hour" is that time:
Between the dark and the daylight,
When the night is beginning to lower
When it is time for children to go to bed. The children are getting ready for bed -- their father hears them getting ready in the room above, and soon they are scrambling onto his lap for a bedtime story.
Yet I know by their merry eyes
They are plotting and planning together
To take me by surprise.
The children are planning to coerce their father into telling them a bedtime story. The the Bishop of Bingen is a character in a story that the father has no doubt told to the children before. It is probably a tale of romance, of knights and princesses that need to be rescued from high "turrets" and "fortresses". The castle imagery is a metaphor for the fairy-tale stores the father has told and the children enjoy. "Blue eyed banditi" is a metaphor for the children - bandits with blue eyes, that are forcing him to tell them a story, but they are no match for him, because he loves them and wants to tell them a story anyway.
He continues with the castle imagery to describe his love for the children"
I have you fast in my fortress,
And will not let you depart,
But put you down into the dungeon
In the round-tower of my heart.
His fortress is his armchair, or his study, and while they are there, he can love them, tell them stories, and keep them in his heart.
Longfellow had five children and was known to be a tender and loving father. I think this comes across in the poem, and it is succesful, don't you agree?