The tower in Gassons field is a water tower. It was gravity fed from the reservoirs in the woodland between Pitham Brook path and the Quenington Road that served Dynevor Terrace. These two reservoir were fed from a pump in the Fairford Mill through pipes across the field and were put in place in the early 20th century.
In the water tower was a large tank under which water tank on a cart was place was filled by pulling a chain/rope to relese the water.
See attached article by Syd Flatman for further information Fairford waterworks