Fatherly Love
This poem is a heartfelt expression of the love a father has for his daughters. He is working in the evening and is interrupted by his playful and merry daughters, but he responds not with irritation, but rather with pleasure and enthusiasm. He responds to them in kind, letting them know they are wanted. He then offers amid his playful comments a comforting and more serious expression of how he will love and cherish them forever, even beyond death.
Time and Space
Time is a bit ambiguous here, and through the frequent references to castles and turrets and dungeons, space is as well. What Longfellow is doing is moving beyond one moment in time and space—an evening in his study—and instead giving a timeless and boundary-less expression of the love between a father and his daughters. Time is mutable and space does not matter because love is more powerful than both.
Youth
What is incredibly charming in the poem is the daughters themselves—they are "merry," have "soft and sweet" voices and their little feet "patter"; they rush and climb and kiss and laugh. They are completely guileless, caring about nothing beyond showing their father love. Youth, then, is a pure and lovely state. It is fleeting, Longfellow acknowledges, but witnessing it is a great pleasure.
