Hola Inma,
Is the use of "ser" in this sentence to indicate where the carnival took place? It caught me out because "It was in Cádiz" sounded as if it was talking about the location (estar), rather than being about where the event happened. If so, I remembered that there is a specific lesson about this in Level A1 "Using Ser not Estar when talking about when / where something takes place." I'm not sure if my thinking is right.
If it is, would it be a good idea to include this lesson in the "All related grammar and vocal list?"
Saludos
John