Yes, in 2026, reservations are essentially mandatory for the major attractions in Sintra, particularly the Pena National Palace. Since 2024, the palace has operated on a strict "timed entry" system to manage the overwhelming crowds and protect the UNESCO World Heritage site. You must purchase your ticket online in advance and select a specific date and time slot for entry; if you arrive late for your slot, you will likely be denied entry. Other popular sites like the Quinta da Regaleira and the Castle of the Moors also highly recommend advance booking to avoid long queues at the ticket offices. It is important to remember that Sintra is about a 40-minute train ride from Lisbon, and the town itself can be very congested; private vehicles are restricted in many areas, so you should factor in time for the local bus (434 or 435) or a steep uphill walk when planning your arrival for your reserved time slot. Booking your tickets at least several days (or weeks for peak summer dates) in advance is the only way to guarantee you will see the interior of the most famous palaces.