Water Fortress, located on the west end of the pier and was the northernmost defence of Batavia City. Because of this, this fortress was also called "Hornwerk" by de Haan. Construction began in 1741, but reformations and improvements were made later on. In 1745, this fortress was laid on a foundation made of hull remains soaked in mud and actually, this type of foundation was not strong enough to hold the fortress walls, which is why in 1750, this structure was demolished and reconstructed.
In the drawing made based on the 3 March 1750 Resolutie, it is clear that the fortress was made of coral reef, equipped with barracks and armed forces with a number of heavy cannons. In the drawing based on the 29 January 1754 Resolutie, it is clear that along the top of the fortress wall, there were about 50 spots for cannons and 8 semi-circular lookout towers, which was called Rondeel by Mohr. There are no buildings in the fortress, whereas the south side of the fortress has a road connected by a bridge to Water Fortress. Actually, the entrance is an extension of the west pier embankment (Ciliwung River Embankment). On the southeast bastion, there were two rectangular plots identified as the fortress commander residence and troop barracks. Based on the scale, Water Fortress was 38 Rijl.Roeden in length or approximately 142.88 m (measured on the longest side). Its width was 27 Rijnl. Roeden or approximately 101.52 m (measured on the widest side).
The objective of this fortress construction was to provide defence at the entrance of Ciliwung because the anchor points had been moved far away from the fortress due to the formation of shoals in front of the estuary and the widening of the coastline.