Wat Si Chum, in the northwestern corner of the park, is the most theatrical temple in Sukhothai. The body of the temple is a 32-metre-square mondop — a squat, windowless brick pavilion 15 metres high. Through a narrow vertical slit in the front wall, you suddenly see what is inside: a colossal seated Buddha in the earth-touching posture, 15 metres tall, its golden hand resting almost casually on its knee. The image is known as Phra Achana, "the unafraid", and it is one of the great sights of Thailand.
Built in the 14th century, the mondop's interior walls were once carved with hundreds of Jataka scenes — many of which are now in the National Museum in Bangkok. The temple is reached by a quiet shaded path through the trees of the park's western zone; arrive early to have it briefly to yourself.