IMPORTANT MESSAGE: NEW CAR RESTRICTIONS IN SINTRA, READ HERE
This December 2019, the city hall started to forbid private cars to reach some parts of Sintra, such as Pena National Palace, Moorish Castle, Pena Park, and others.
Private cars such as rental cars, foreign cars, and national cars, are not allowed to enter the restricted area. It will be required to park the car outside Sintra central area, and take public transportation to the palace/castle. Taxi, Ubers and Tourism Companies are allowed to enter the restricted area.
Here is the official map with restrictions: https://www.google.com/maps/d/u/0/viewer?mid=1G43FrMuAc3iNYPmQRAICBKqw2za7gIW6&ll=38.7918322167363%2C-9.39508324999997&z=15&fbclid=IwAR0SEsOnauAgOEQv7K1ZogA-xlY7b9MXZMDQ28sgrjnU1aSquSfivgyOffs
Here is the official announcement, with information about public car parks outside the center: http://parking.sintra.pt/en/