Very cool apartment! Being an old cellar means it is a very unusual interior compared to normal, and is all the better for it. Be aware though that although the apartment is very spacious, there aren't any internal doors! Could be important if quiet/private areas are needed. That being said, there was two of us staying, and we just went opposite ends of the apartment and we couldn't hear each other snoring.
The location is brilliant; only a few minutes walk to the cathedral and palace, with loads of non-touristy cafes and bars just nearby. Super easy to get to with metro stops just a few minutes walk away too.
Carlos was always quick to respond whenever I asked anything.