The Golden Mount is an artificial hill constructed under King Rama III (1824-51). When the king tried to build a chedi here before the hill existed, it collapsed because of the soft soil beneath. He therefore made a strong pile of mud and bricks to support it.
The mount was started by King Rama III, although what you see now isn't what he had in mind. The plan was to build a giant chedi - a much larger version of the golden bell-shaped shrine you can see at the Temple of the Emerald Buddha. The core structure was being constructed using logs and bricks. The soft earth of Bangkok could not support the weight and the pagoda collapsed before it was completed. The project was abandoned until a later king built a small chedi on top of the mound. The concrete casing seen in the photo was only added around World War II.