Katia's place is great- her description of the apartment is very accurate and it was a lovely place to call home base for our time in Stockholm.
The kitchen and bathroom are very modern but the apartment itself has lots of old world charm. If you are visiting in the summer and not used to the long daylight hours, I'd recommend an eye mask- the skylights allow lots of great natural light in during the day, but if you're not used to it, it can be hard to fall asleep.
The roof deck was so fun and we really appreciated the little things like coffee and filters in the kitchen, umbrellas in the hall, and extra pillows and comforters for sleeping. A great place if you're hoping to spend some time in Gamla Stan!