Fort Tienhoven in the Tienhoven polder was built in 1848-1850. The fort consisted of a defensible guardhouse that was later converted into a bombproof barracks. It served to close off the Tienhovense Vaart and to guard the adjacent inundation quay, dam sluice and crow's nest lock. In the period between World War I and World War II, a cast-steel turret casemate for a heavy machine gun was installed to cover the Tienhovense Vaart and Kade.
The fort has been returned to nature. Today, bats feel at home there. It is not open to the public (yet), but the surrounding area is regularly refurbished, so you can see the fort from the outside. It lies along a walking and cycling path. In the future, a new function may be given to this special fort island.