This is a fabulous location in a great city. Mercedes is a wonderful host and is genuinely keen to help you explore the treats on offer, whether that be tapas bars, nightlife, the historical buildings and architecture, or whatever takes your fancy.
But travel light! There are 108 steps up to the apartment and no lift. Which might be a test if you have heavy luggage. That said, once you get to the top, it's the Airbnb paradigm of not just staying somewhere, but living in a quintessential Barcelona residence in an unbeatable location.
Also make sure that you arrange (if relevant) to book the friendly taxi from the airport through Mercedes. Very god value!