By the time of their fifth album, Led Zeppelin were established living musical legends with nothing left to prove. So they went and invented things to prove with music pulled from somewhere that was not previously physical reality. The Houses of the Holy album was a new row of elements on the musical periodic table.
The lead track on the album is "The Song Remains the Same." That title was given to a song that was more unlike a typical rock "song" than anything that had preceded it. It's closer to an orchestral composition with its development of ideas.
In fact, the four songs comprising Side A of Houses of the Holy fit pretty closely with the traditional concept of a classical symphony. "The Song Remains the Same" stands as the first movement, in something close to the typical sonata-allegro form. The second track, "The Rain Song," is exactly the type of adagio you'd get in a symphonic second movement. The legendary "Over the Hills and Far Away" as the third movement doesn't conform to a minuet or scherzo, but third movements tend to be the most flexible in style, and the song’s middle development does hit a notable scherzo-like three-beat. "The Crunge" is a textbook rondo for the fourth and final movement.
I've never heard anyone describe Side A of Houses of the Holy as an attempt at a rock symphony, but the structure seems to speak for itself.
"The Song Remains the Same" is one of the most impressive, complex songs a rock band has taken to time and effort to compose. The studio production is almost negligible; there are no super special effects calling attention to themselves. The magic is in the notes being struck. The 12-string guitar part is worthy of lore. The extended instrumental sections and especially the guitar solo represent the high water mark of Jimmy Page's ideas. John Bonham's drums are dynamic and irreplaceable; their flying pace creates a strange sense of heaviness in a song that is far more psychedelic in tone otherwise. There's not really any standard genre descriptor I can come up with to describe this song's effect. I can't even think of a practical purpose for the song beyond being a necessity for the ear.
The lyrics are some of Robert Plant's finest abstractions, glints of meanings reflecting in a hall of guitar mirrors.