You're not wrong about Renaissance musical styles. A lot of composers, particularly church composers, used plainsong tunes extensively in their works. Some (early Tallis, for instance) are basically harmonising and regularising older plainsong tunes, some of which can be a bit boring to the modern ear. Some others take a plainsong tune as a cantus firmus, a long, slow tune with magnificent flights of polyphony winding around it. Actually they didn't stick with just plainsong for that; there seemed to be something of a competition between composers as to who could sneak the most ribald folksong past the priest as a cantus firmus.

Sorry if I wibble, I got seriously into early music in my late teens and early twenties as a result of a friend. I sort of dropped out of it; Robert is still running the now pretty famous choir (http://www.ifagiolini.com/) he started in college!