This flat has a great location with loads of shops and restaurants within walking distance. The castle is a 15 to 20 minute walk (we had little ones). Part of it is through a steep hill, but you get lovely views of the castle and of the Princess Street Gardens. If you have children, there's a playground at the Gardens within a 5 minute walk.
The flat is spacious and accommodated our family of 4 adults and 3 children (one baby in a provided cot) really well. It is an older flat, so as some people mentioned, the floors make quite some noise, but it wasn't an issue for us.
It was very clean and enough toiletries and towels were provided.
You do have to climb a set of stairs, but they were not an issue, even when carrying the strollers up.
Overall a great stay at a beautiful city.