When Juan Ruiz wrote El Libro de Buen Amor, the language of the time was Old Spanish, also known as Old Castilian. This early form of Spanish was before any type of consonantal readjustment was introduced into the language system.
For example, the voiceless velar fricative /x/ did not exist in Old Spanish. Previously in Old Spanish the phoneme /ʒ/, written with the grapheme “x” would evolve into the Modern Spanish phoneme /x/ written with the graphemes “g” or “j”
> Example: Old Spanish DEXA [deʒa] evolves to Modern Spanish DEJA [dexar]
Similarly the Old Spanish the phoneme /ʒ/, written with the graphemes “jo,u,a” , “i”, “ge,i” would later evolve into the Modern Spanish phoneme /x/ written with the graphemes “g” or “j”
> Example: Old Spanish ESCOGE [eskoʒe] evolves to Modern Spanish ESCOGE [eskoxer]
The Old Spanish affricate voiced alveolar phenome /dz/ was written with the grapheme “z”, which later evolved into the Modern Spanish fricative alveolar voiceless /s/ which is represtned with the grapheme “s” or in some seseantes varieties “ce,i” or “za,o,u”
> Example: Old Spanish FAZE [fadze] evolves to FACER [faser]
The Old Spanish affricate voiceless alveolar /ts/ was written with the graphemes “ça, o, u” and “ce,i”, then later evolves into the Modern Spanish interdental voiceless fricative /θ/ or the fricative alveolar voiceless /s/ for some seseantes varieties
> Example: MARÇO [martso] evolves to MARZO [marθo] or [marso]
The video below demonstrates how the pronunciation of Old Spanish deviates from our modern day Spanish.