The San Paulo Building (San Paulo) is the historical ruin of an astronomical observatory and Jesuit church built during the reign of King Narai the Great. The architecture featured a prominent three-story octagonal tower, used by French astronomers and priests to observe the sky and celestial bodies. The building was constructed with thick brick and mortar using Western engineering, featuring arched windows that blended European style with Thai craftsmanship. Although only the foundations and partial walls remain today, it stands as vital evidence of the first international-standard observatory in Southeast Asia. The local name San Pao Lor is a Thai phonetic variation of Saint Paulo, named after the prominent Christian saint to whom the site was dedicated.