Sintra served as a mountain retreat for Portugal's royals and elites for hundreds of years and as such, is now a UNESCO World Heritage Site full of castles, palaces, forts, and mansions. Our second day in Lisbon, we decided to take my co-worker's advice and hop the train to Sintra. There's way too much to see in Sintra on just a day trip though, and we quickly became overwhelmed trying to figure out which sites to visit. However, we kept seeing a place called Quinta da Regaleira and it had the best reviews out of everything in Sintra (five stars from 9,829 reviews on TripAdvisor) so we were like obviously we have to go there!