If you have a problem with your back, please be aware that mattress is unbelievably hard at the flat. I don't have any problems with my back and normally don't have any pains in my back however all 3 nights didn't slept well and was waking up with pains on my back. It felt like sleeping on a floor. Also, there were 3 of us and even though I wrote to the owner that it's me, my sister, and my mum, we still got only 2 duvets to share between 3 of us. Which wasn't a big problem but something to keep in mind if you are traveling with friends and don't want to share a duvet. What's more, the flat is a bit noisy because it's on a little junction and apparently Portuguese drivers are not very patient people so there were beeping noise, especially in the early morning during the week.
But apart from those little things to keep in mind, the flat was good and in a lovely old area. And there is a lovely vegan cafe literally just around the corner!
After all, I recommend this flat and Lisbon (and Sintra!) itself :)