The apartment has been renovated quite recently, hence everything is new. It is quite big and is certainly convenient for a family of 5 people. It is very well equipped, nothing is really missing.
If you come by train from Lisbon, it is 2 min walk from the station. Despite the station being so close, there's not a lot of noise and you can have quiet nights.
If you come by car, it is a bit more complex to reach the apartment because there's a lot of traffic in Sintra and a lot of streets are one-way streets. Meanwhile, once you get here, you can really park in from of the apartment.
The apartment is also very close (2-3 min walk) from the departure of busses going to Palace of Pena.
Finally if you like walking, the Palace of Sintra is at 10'-15' and a few other curiosity are at walking distances.