Having rented many apartments in Spain over the years, I can honestly say that I've never been asked to pay a "key fee". I have seen advertisers add a cleaning fee to the rental price and have also noticed that others may ask for an advance payment for damage, returnable if the apartment is found to be left in the same condition as it was before the occupancy began. I personally wouldn't rent an apartment where any of these "add-ons" are asked for.
All I want when renting is a price for the length of the stay. I assume the owner has arrangements in hand for a cleaner to attend, which of course they have and in general the fee has included a payment to this person who, besides cleaning, also ensures the accommodation is prepared for the next occupancy.
If Airbnb haven't described this rental in full, and hidden costs have appeared, then you should be entitled to a refund of any money you have paid them to date. In order to protect their reputation I would have thought that such a refund would be forthcoming without any fuss but, of course, this isn't always the case. In the event that your money is returned I would tend to look around at rental prices for accommodation in Spain as, from my experience, you could get far better deals elsewhere.
The following would seem to match your situation and may be of assistance -
https://www.airbnb.com/help/question/251
Good luck.