In Late Latin and the early stages of Castilian, prepositions assumed
a role that they did not have in Classical Latin; namely, the identification
of the case marks of the now disappeared flexional case system.
This new role led to the grammaticalisation of the most commonly
used prepositions and, thus, the progressive disappearance of the systematic
set of semantic oppositions of Latin. The highly systematic set
of prepositions was superseded by a chaotic collection of prepositional
items in Spanish.