The consonantal system of Castilian Spanish, by the 16th century, underwent the following important changes that differentiated it from some nearby Romance languages, such as Portuguese and Catalan:
~ The initial /f/, that had evolved into a vacillating /h/, was lost
in most words (although this etymological h- has been preserved in spelling).
~ The voiced labiodental fricative /v/ (that was written u or v) merged with the bilabial oclusive /b/ (written b). Orthographically, b and v do not correspond to different phonemes in contemporary Spanish, excepting some areas in Spain, particularly the ones influenced by Catalan/Valencian and some Andalusia.
~ The voiced alveolar fricative /z/ (that was written s between vowels) merged with the voiceless /s/ (that was written s, or ss between vowels).
~ Syllable-final [m] is an allophone of /n/ before bilabial consonants /b/ and /p/.
The consonantal system of Medieval Spanish has been better preserved in Ladino, the language spoken by the descendants of the Sephardic Jews who were expelled from Spain in the 15th century.
Spanish has a phonemic stress system — the place where stress will fall cannot be predicted by other features of the word, and two words can differ by just a change in stress. For example, the word camino (with penultimate stress) means "road" or "I walk" whereas caminó (with final stress) means "he/she/it walked". Also, since Spanish pronounces all syllables at a more or less constant tempo, it is said to be a syllable-timed language.