Sintra is a town of palaces. The National Palace at the centre, with its two distinctive conical chimneys, is the oldest — the Moorish structure reworked by every Portuguese king for 600 years. High above on the ridge sits the Pena Palace, the 19th-century romantic fantasy commissioned by Ferdinand II, painted in yellows and reds and visible from miles away. Below Pena, the ruined Moorish Castle walls snake along the ridgeline.
The other must-see is Quinta da Regaleira, an eccentric early 20th-century estate full of underground tunnels and esoteric symbolism. Beyond these, the Monserrate Palace and the Convento dos Capuchos (a tiny monastery of cork-lined cells hidden in the woods) are worth the extra time if you have it. Do not try to see everything in a single day — you will queue yourself miserable. Two days, starting early, is the right amount.