The apartment of Carlos is definitely worth a visit. It had everything we needed.
Sadly we didn't meet Carlos or Christina personally, but their father welcomed us perfectly when we arrived at the Ciutadella. He gave us a short look around the apartment and told us where we could find a grocery and the way to the center of the city.
We were flashed by the apartment, as it is super clean, has a kitchen including everything you could probably need on your stay (even some topper ware!), a wonderful roof terrace, a sunshade and a cool bag (perfectly useful for a trip to the beach). On the upper floor there a two rooms with beds and a bathroom with a washing machine and a hairdryer. So there is nothing we missed during our stay.
They even provided some informations about the city Ciutadella and some places on the island to us.
Whenever we had question we could contact them and they provided a solution for us.
The apartment itself is placed in a quiet neighbourhood next to the city. It took us only 10 minutes from it to the centre of the city (plaça des born) and around 15 to the port. There are a few grocery stores nearby and 3 big ones (Lidl, Mercado) in the near industrial estate. In front of the apartment there are parking places on the sides of the street, you can use.
We would definitely recommend the apartment and are planning to come back ourselves, too.