A repetitive opening bit builds into some nice guitar harmonies, then breaks into another repetitive but more memorable riff.
The verse vocals are a mess, trying to stuff in too many syllables while following the guitar melody, but Paul isn't fully in time with the guitars, and isn't always delivering as many syllables as there are notes.
I guess the follow-up section with harmony vocals is the chorus, and it works a lot better than the verse, but it's over pretty quickly.
Another round of verse and chorus, and then we get a staccato bridge with some decent solo vocals and cheesy vocal harmonies before heading into a tasteful solo section.
The song stops, then comes back in with a bass riff and some nice supporting guitar harmonies, building up a bit until it breaks into what will become the iconic Maiden gallop and chord progression. Some nice harmonized leads come in and carry us to a pretty good pair of solos, then back to the harmonized leads, and then to a more boring solo lead. One more verse and chorus and we end on a solid, sudden note.
I know this is a generally beloved song, and it's the first time we hear what will become the hallmarks of the Iron Maiden sound; but almost all of the good stuff is in the second half of the song, and the rest is kind of a mess. 6/10.