The check in and check out were easy, and we were able to contact various people from the apartment rental agency with questions during our stay, who responded quickly in general. The apartment is well located for those who want to enjoy the beach (a 5 minute walk), a bus in front of the building goes to the city centre, and the metro is a 10 minute walk. The kitchen is well equipped (washing machine, dishwasher, cooking utensils) and the private rooftop is a plus (one floor up, not accessible directly from the apartment, but from the building’s main staircase).
The main problem with the apartment was the beds: the springs of the double-bed mattress stick out and are uncomfortable, and the bunk beds are rather unstable meaning that they make a lot of noise at the slightest movement - the top bed is almost unusable (my friend took the mattress and slept on the floor instead). Guests should also note that the sofa is really just a sofa, and not a sofa bed at all.
The other problem is with the windows and doors: there are no locks on the windows, so the metal shutters must be closed each time you leave the apartment, and the door between the kitchen and small balcony is impossible to close so remains open at all times. Fixing these things would improve the comfort, the sleep and the security of guests.
Overall, we enjoyed staying in this apartment, and little more maintenance would make it a 5 star stay.