Beautiful, quiet and comfortable apartment, would highly recommend. I stayed for a week while visiting for my friend's wedding/working holiday. It was perfect walking distance to the castle gardens, the castle, Charles Bridge, trams and metro, and the main square in Mala Strana. Walking to old town only takes about 15 minutes. The apartment is quite large and has a really nice kitchen and outdoor patio area. The WiFi worked great in the bedroom and the kitchen, which is where I worked out of, though didn't extend very well into the living room.
Kate was extremely communicative when I needed to change my dates. As others have said, there's no washing machine, but when I needed recommendations for laundry services (and a nail salon for the wedding), Kate was really helpful! It was quite hot while I was there and there was no a/c but there was one fan provided which was helpful and worked fine since it was just me. If you have someone staying on the sofa bed you may want to request an extra fan during summer months. All in all one of the nicest Airbnb's I've stayed in and would highly recommend.