History of Valenzuela City
https://www.discoverphilippines.net/2010/05/history-of-valenzuela-city.html
During its long, colorful history, the area where the present-day Valenzuela City is located was considered a flourishing agricultural and fishing town. It is part of a huge land area that also include parts of present-day Quezon City, Novaliches and Obando, Bulacan, which was named Polo during the Spanish era.
Polo is derived from the Tagalog word "pulo," which means "island." Several bodies of water bound the area - the Tullahan River, which connects to the Pasig River, on the south, and a number of connecting rivers, including the Rio Grande de Pampanga, on the north. Because of the fact that these rivers surrounded the area, early inhabitants considered it as a separate island or "pulo" that was later changed to the more Hispanic "polo" during the Spanish rule.
These first settlers have long resisted Spanish rule. Polo natives joined a series of epic battles against the Spanish forces led by Miguel Lopez de Legazpi, who were dead set on conquering Rajah Sulayman's Maynila kingdom, which also covered Polo, Tambobong (now Malabon City), and the mountains of San Mateo. Sulayman, who enlisted local warriors and seafarers from his kingdom and neighboring Bulacan province, engaged the Spaniards, led by Legazpi's nephew Martin de Goiti, in the infamous Battle of Bangkusay on June 3, 1571. Sulayman's forces were vanquished, with the Maynila king killed in battle. After destroying Sulayman's army and forging friendly ties with Rajah Lakandula, the Spaniards captured Maynila and the neighboring towns, including Polo. The vast sitio was then incorporated into the township of Catangalan (now Meycauayan) in the Bulacan alcaldia (local government).
Seventeen years later, local leaders plotted what was considered as the first revolution against Spain. In 1587, the Catangalan cabeza de barangay, Tassi Bassi, and Polo chief Felipe Salonga, joined the revolutionary forces led by Magat Salamat in what was considered as the "Tondo Conspiracy of the Maharlikas," a planned insurrection that included kin-related noblemen or maharlikas based in Tondo, Polo, Pandacan, Candaba, Taguig, Misil, Caranglan, Navotas and other localities in Manila. However, this plot was discovered and quelled by the Spaniards, who got the information from an informer. Interestingly, this was unintentionally repeated three centuries later when Andres Bonifacio established the Katipunan, which the Spaniards also discovered and quashed. This revolution was also supported by the local intelligentsia and political leaders, including Dr. Pio Valenzuela, whom the present-day Polo is named after.
After the Spaniards established the Manila archdiocese on August 14, 1595, the friars who had set up churches in Catangalan called for further division of the vast town to enhance efforts to convert locals into the Catholic faith.
In 1623, Spanish governor-general Alonso Fajardo de Entenza ordered the separation of sitio Polo from Catangalan and its establishment as an independent town. Both of these towns, however, still fell under the Bulacan alcaldia.
The first Polo cabeza de barangay was Don Juan Monsod, who worked with Spanish friar Juan Taranco to make the separation from Catangalan possible. Taranco operated the parish of San Diego de Alcala, with the construction of the parochial church started in 1627 and completed two years later.
The San Diego de Alcala Parish is located at present-day Barangay Poblacion, the center of the old town of Polo. The church ruins seen today the belfry and entrance arch serve as a reminder of the 376 year-history of the town.
Since its establishment, the town of Polo served as a Spanish garrison, friar hacienda and political settlement.
Source: Valenzuela Government