I had lovely stay in Sintra here, and it's in a wonderful location with just about everything within walking distance.
The living space was very neat, and from the booking until arrival Mariana was fantastic with communicating clearly about what to do for check-in, and there were some coffee pods and cookies as a courtesy left for me, which was lovely.
I do echo the other comments (which I missed before booking) about ceiling height - I am 6'1" and I was unable to stand upright when facing the bathroom sink, due to ceiling slope - and also bonked my head coming from hallway into main living area, not ducking low enough - it's just a good caution for anyone over 6' to be aware of, for those parts of the apartment (bathroom / hallway entrance)
There are also upstairs neighbours that can be heard talking or walking around above - and on the first night I did put in earplugs to get to sleep as they were up late - but it wasn't excessive noise or anything - just unexpected as the rest of the surroundings are so quiet.
Overall I highly recommend for a solo traveller or a couple - very compact space great to rest in and maybe have a few modest meals, instead of eating at restaurants. (fridge is mini and very little space if you wanted to prepare anything elaborate - but perfect for regular things).
Would gladly stay again if ever in Sintra.