i would say this song suffers most from lack of drama.
you want some sort of thematic conflict to keep things interesting.
remember- this song is for your wife... your soulmate. she's seen you at your best and at your worst. she's seen you pick your nose, she's seen you on the crapper. it's likely that, depending on how long you two have been together, she's unlikely to be surprised by anything you do.
this is why you must take this opportunity to surprise and excite her more than anything you've ever done before... to remind her of why she fell in love with you in the first place.
the castle part is good and i'm sure it can be worked in but as a general theme, i recommend dragons (fire-breathing are better... there's a certain... titilation about fire-breathing dragons). start with a storyboard. you are a fearsome warrior, she is a beautiful princess... that type of thing. of course through the course of the song you must win her love by rescuing her from the dragon.
i envision something grand in scope, with perhaps a lengthy instrumental section and an intense choppy part where you finally fight (and ultimately slay) the dragon. this should obviously happen somewhere toward the end of the song. sample lyrics from such a section could be as follows:
...and the mighty prince
with a heavy heart and fire in his loins
shall slay the mighty beast!
and triumph, to say the least!
WITH MIGHT AND SIGHT HE'LL FIGHT FOR WHAT'S RIGHT! (x32)
the might and sight part, maybe you could cut back to only repeating 16 times depending on the structure of the fight sequence, but i urge you (and give you full permission) to milk that line for all it's worth because i believe it to be lyrical gold.
the dragon's death could be accented by a short spacey section (the musical "dust clearing") and then, of course, you'd roll right into the celebratory end.
remember... nothing is too ambitious for your wife.
when she hears it she will totally be like what the fuck.
