The apartment is amazing. It has all that you may need, it's located right at the heart of Gamla Stan and even so, it's still super quiet and relaxing.
The only warning for other travelers would be to bring a sleep mask if you plan to use the third person bed (which is an inflatable bed that's placed in the living room) since that room has no curtains and there is a street light that shines directly into it at night.
We knew that it would likely be the case (judging by the apartment photos) and brought one. Using it, I slept there perfectly.
Fredrik is a great host. Super attentive and with immediate responses, he'll make your stay there as pleasant as it can be.