Martin's apartment is in the center of Edinburgh, right next to the Grassmarket and right below the Castle. All tourist attractions are easy to walk to. At the same time, the apartment is in a quiet little side street, and I didn't hear any noise.
The apartment is very nicely equipped, with a Jacuzzi, Netflix and some free food in the fridge. Martin's hospitality was outstanding, and he even gave me a detailed description of the places that would be worth a visit.
All in all I can fully recommend to stay with Martin.