Wat Chaiwatthanaram is one of Ayutthaya's great temple ruins, set on the west bank of the Chao Phraya with a central prang and surrounding towers that show strong Khmer influence. Built in the 17th century under King Prasat Thong, it has the scale and silhouette that many travellers imagine before arriving in Ayutthaya.
It is especially good late in the day, when the brickwork catches warm light and the riverside setting opens up. Do not skip it just because it sits off the main island; this is one of the historic city's essential stops.