I saw this on wikipedi:
"Bonham's drums featured a phasing effect (giving the bass drum its heavy, thundering "thump" sound) courtesy of an early Eventide phaser supplied by engineer Ron Nevison. [4] Plant stated that Bonham's drumming is the key to the song: "It was what he didn't do that made it work". [2] Sections of the song utilize a polymeter effect, with the drums and lyrics in quadruple meter while the melodic instruments play a triple meter rhythmic pattern"
Orchestral brass and strings with electric guitar and mellotron strings appear in the song. This is one of the few Led Zeppelin songs to use outside musicians. Session players were brought in for the string and horn sections. [3] According to Jones, "the secret of successful keyboard string parts is to play only the parts that a real string section would play. That is, one line for the First Violins, one line for Second Violins, one for Violas, one for Cellos, one for Basses. Some divided parts [two or more notes to a line] are allowed, but keep them to a minimum. Think melodically".
http://en.m.wikipedia.org/wiki/Kashmir_(song)#_