Location, location, location!
This was a very important decision factor for us, and the flat is indeed very well located. It was well cleaned and one of the windows had a nice view of the castle.
The hosts were great giving us tips about what to do and what to avoid, they even helped us climb our luggage up the stairs. Yes, the flat is in the 4th floor and taking our luggage up and down was no easy task, specially because my sister is visually impaired and need assistance as well. I wish I knew in advance, so be aware of that before booking.
Another thing to keep in mind if you are going during the summer is that the apartment is very hot and there's no AC. It's very exposed to sunlight during the day and the thick walls radiate the head during the night. The flip side is that it should be excellent in winter.
The street is very busy and loud, but the windows provide very good isolation (if you can keep them closed - see temperature above)
The bathroom is really tiny so if you are 6'2" like myself prepare for some contortionist.
Since we spent most of our time out all this were minor points to our overall experience and I would recommend the place for someone who can deal with this drawbacks.