Phra Sri Ratana (Si Rattana), the main stupa of Wat Phra Kaeo, was built by King Rama IV (Mongkut) in 1855 to house a relic of the Buddha. Its exterior is covered with gold mosaic tiles. The stupa's form, with its four directional entrances, imitates the much earlier chedis of Wat Phra Sri Samphet in Ayutthaya.