Yes, booking Pena Palace in advance is absolutely essential in 2026. The palace operates on a strict timed-entry system to manage the overwhelming crowds that flock to Sintra. If you show up at the gate without a pre-purchased ticket, you will likely find that all slots for the day are already sold out, or you may be left waiting for several hours. By booking your 2026 ticket online via the official "Parques de Sintra" website, you can secure your preferred time slot—ideally the 9:00 AM opening slot—to avoid the heaviest midday heat and tourist bus arrivals. Remember that your ticket time is specifically for the entrance to the Palace building, not the park gates; you should arrive at the main park entrance at least 30–45 minutes before your palace time to allow for the steep uphill walk or the shuttle ride to the summit.