Marta's apartment was perfect for me and my partner's stay in Barcelona! We loved the view from the balcony (you can see Sagrada Familia!); the location was perfect for walking to the Gothic Quarter, the beach, and the park; and it felt very safe. It was about a 20 minute walk to most major attractions while also being close to a metro station.
Just be warned: there are A LOT of stairs. Like a lot. I think we counted around 200. If you have any issues with that, this may not be the place for you.
Marta was also super quick to respond to any questions and was an excellent host. I would stay here again without a second thought. You should too!