There are two interesting compositional techniques which are used in this song. They also appear in other Maiden songs, but they are especially prominent here.
1. Rhythmic modulation (also known as metrical modulation). In general, this term refers to a tempo change in which the new tempo is based on some rhythmic element of the old tempo. In this song, the fast and slow sections are related by a ration of 2 to 1. That is, two beats in the fast sections become one beat in the slow section. This is a common type of rhythmic modulation, and in these cases the slower sections are sometimes called 'half-time' - or, conversely, the fast sections can be called 'double-time'.
2. Pedal points. In general, this term refers to the bass remaining on a single note while the melody instruments (guitars in this case) change chords. Two clear examples in this song are:
a ) "The killer's breed or the demon's seed..."
The bass remains on an A note while the guitars change chords.
b ) The closing of the guitar solo section
The bass remains on E as the guitars play a C-D-E progression.
