Fort Zeelandia (Chinese: 熱蘭遮城; pinyin: Rèlánzhē Chéng; Pe̍h-ōe-jī: Ji̍at-lân-jia Siâⁿ) was a fortress built over ten years from 1624 to 1634 by the Dutch East India Company (VOC), in the town of Anping (now Anping District of Tainan) on Formosa, the former name of central island of Taiwan, during their 38-year rule over the western part of the island.

As trade at the time depended on "military force to control the markets", the value of Formosa to the Dutch was mainly in its strategic position.

"[1] On behalf of the VOC, ships departing from Formosa could head north to Japan, west to Fujian, or south to Vietnam, Thailand, Indonesia, Cambodia, India, Iran or Europe.

In August 1624, the Dutch were expelled from the Pescadores, having failed at their attempt to use military force to coerce Ming China into trading with them.

[4] As their first priority was to strengthen defenses, the Dutch built a temporary fort on a raised sandy bank at the entrance to Taoyuan harbor, off the coast of modern-day Anping District, Tainan.

[2] While of solid construction, the fort and its siting were not so much for the purpose of defense against a major enemy as they were for defending against the islanders and for overseeing trade.

On 30 April 1661, Ming dynasty-loyalist Koxinga laid siege to the fortress (defended by 2,000 Dutch soldiers) with 400 warships and 25,000 men.

Bricks were brought from Java and the mortar consisted of a mixture of sugar, sand, ground seashells and glutinous rice.

The fort was designed to be surrounded by three concentric layers of walls and its four corners were built into protruding bastions for better defense.